Terminology and nomenclature of protist cell surface structure

These definitions provide precise terminology on the different terms used for cell surface structures of protists and related organisms. For cross-references the abbreviation q.v. ("quo vide") has been used. To view the scientific paper from which these definitions were drawn (and the accompanying illustrations) see H.R. Preisig et at. (1994), Terminology and nomenclature of protist cell surface structures, Protoplasma 181:1-28.

 Acellular stalk. see Stalk.

 Adhesions. Morphologically and chemically diverse classes of compounds which function in attachment of cells to substrates or hosts. Distributions of adhesions at the cell surface range from adhesive junctions to the entire cell surface. Symphoriontic ciliated protozoa, for example, attach to host carapaces with stalks (q.v.) that have adhesive bases. Fine pseudopodia in amoeboid protists that adhere closely to the substratum are categorized as adhesive. See also, Adhesion pad, Holdfast adhesion, Holdfast organelle.

Adhesion pad. A type of extracellular matrix (q.v.) which spreads at the cell or spore substrate interface, binding the two surfaces together. "Adhesion pad", "adhesion plaque", "mucous pad" and "spore tip mucilage" are used as synonyms. The polarized distribution of this material distinguishes it from an extracellular matrix or sheath (q.v.). Among oomycetes it attaches encysting zoospores to a host or substrate; among pennate diatoms it denotes distinct, localized regions of polysaccharide adhering cells together into colonies; among pedinellid flagellates (such as Apedinella) it is a surface associated structure that spine-scales attach to. Adhesion pads may have diverse origins including: (1) discharge from pre-existing organelles such as K-bodies as oomycete zoospores are induced to encyst and (2) selective secretion of polysaccharide from localized pores on the frustule (q.v.) surface of diatoms. Adhesion pads of diatoms are a specialized part of the non-silicified, or­ganic casing and maintain connections between daughter cells following division. The exact position of these pads determines the overall morphology of the colony. An adhesion disc made of denticles (ring) immediately beneath the pellicle (q.v.) occurs in the ciliate Trichodina. See also, Adhesions, Holdfast adhesion, Plaque, Stalk.

Adhesion plaque. see Adhesion pad.

 Alveoli. Membrane-bounded flattened vesicles or sacs underlying the plasma membrane (q.v.) in certain protists, most notably, the (majority of the) ciliates, the dinoflagellates [where they are called "amphiesmal vesicles" (q.v.)], some sporozoan (apicomplexan) parasitic protists, and possibly the glaucophyte algal protists. A single alveolar vesicle is also known as a "lacuna", and the entire system in ciliates has been called a "lacunar system" in older literature. In more recent usage, it is called pellicular alveoli. A similar system found in the Sporozoa is known as "inner membrane complex". In some actinopods (e.g., radiolaria) an organic capsular wall (q.v.) surrounding the central cell mass is deposited within lacunae resembling alveoli. The flattened peripheral cisternae underlying the plasma membrane of oomycete zoospores may well deserve consideration as a kind of alveoli. In diatoms, the term alveoli is used for elongated chambers of the valve (q.v.) opening to the inside of the cell by a large opening and with a perforate outer layer. See Conclusions and recommendations; see also, Cortex, Epiplasm, Pellicle.

 Amphiesma. Term describing the plasma membrane (q.v.) and the underlying flattened vesicles of dinofla­gellates (amphiesmal vesicles = "thecal vesicles") which in some species contain plates (= thecal plates). In certain naked dinoflagellates the amphiesmal vesicles appear to be without plate-like structures. Apparently the same system of vesicles underneath the plasma membrane occurs in ciliates, some apicomplexans, glaucophytes, and perhaps other protists (see Alveoli), but the term amphiesma is presently restricted to dinoflagellates.

Amphiesmal vesicles, see Amphiesma.

Appressorium ring. Extracellular matrix (q.v.) in an o-ring like arrangement through which the penetration peg from the appressorium of some pathogenic fungi, such as rust fungi, passes just prior to entering host cells. Believed to assist in attachment of appressorium to host cell and preventing dislodgement as host colonization occurs.

Auxospore wall. The cell wall (q.v.) of a diatom auxospore (a cell resulting from sexual fusion or autogamy, or one formed purely by vegetative processes that swells to maximum size). The auxospore wall differs from that of the normal vegetative cell. Its primary wall consists of polysaccharide material, which in most centric diatoms contains overlapping siliceous scales (q.v.). Auxospores may also develop a secondary wall system consisting of siliceous hoops or bands, collectively termed the "perizonium" (in pennate diatoms) and "properizonium" (in centric diatoms), respectively. Unlike the perizonium, the properizonium is not separated spatially from the primary auxospore wall, but is physically and developmentally continuous with it. The individual siliceous components of the perizonium (properizonium) resemble modified girdle bands, and are secreted sequentially to create the secondary auxospore wall. The new frustule (q.v.) of the enlarged initial cell is deposited within the perizonium (properizonium), which then ruptures to release the cell.

Basal disc. A general term describing the attachment of the stalk (q.v.) (or occasionally the entire aboral surface of the body) to a substrate in a wide range of protists. See also, Hypothallus.

Basket. (1) An enclosing structure composed of loosely arranged components [spicules, costae (q.v.), etc.] surrounding a cell as in some choanoflagellates and silicoflagellates; (2) a type of complex scale (q.v.) on the surface of some flagellates; (3) the conical array of microtubules forming a feeding apparatus in nassulid-type ciliates. The feeding apparatus in nassulid-type ciliates is also known as a "feeding basket", "cytopharyngeal basket", or "cyrtos" and is an intracellular structure with the narrow end of the conical array of microtubules pointed toward the interior of the cell. It is a dynamic structure for grasping and ingesting food particles, including filamentous algae in some species. See also, Lorica.

Bristle. Siliceous elongated structure that is tucked under, and cemented to, the distal portion of a scale in the synurophyte genus Mallomonas. The term "bristle" is also widely used to describe diverse stiff hair-like appendages on cell walls and other surfaces (e.g., in some chlorococcalean algae). In ciliates it is a common name for stiff cilia of several kinds and functions. The terms "bristle" and "spine" (q.v.) are sometimes used as synonyms (e.g., in ciliates). Bristles of Mallomonas radiate outwards from the cell and may change their position relative to the longitudinal axis of the cell. They are streamlined when the cell is actively swimming and positioned closer to 90° at rest. Movable bristle-like components, possibly homologous to bristles of Mallomonas, also occur on the valve (q.v.) of the diatom Corethron. Euplotid ciliates have modified cilia known as bristles on the dorsal surface; they may be sensory.

Capillitium. Sterile threads, which are often coiled and bear ornamentation, intermixed with spores of fruiting bodies of some Mycetozoa. Differentiated prior to complete spore cleavage, these threads aid in spore dispersal. Among higher fungi the term is not restricted to extracellular material, but can describe specialized hyphae in frutifications such as those in puff-balls. The term "elaters" is sometimes used for capillitial strands that are short and unbranched.

Capsular wall. An organic wall in actinopods, especially radiolaria, composed of closely spaced plates deposited within cytoplasmic lacunae or alveoli (q.v.) at the periphery of the central, more dense cytoplasm. Cytoplasmic projections protrude through pores in the wall. In myxozoan spores, the polar capsular wall consists of two layers with varying sensitivities to chemical digestion which can be observed with transmission electron microscopy; the inner electron-lucent layer resists alkaline hydrolysis and the outer electron-dense layer is digested with proteases. The thickness and organization of the wall material varies among actinopod species, and the penetrating cytoplasmic strands vary in complexity and function among major groups. See also, Capsule.

Capsule. A perforated organic layer surrounding the cell body of heliozoa, but not necessarily enclosed within a cytoplasmic envelope. Capsule and capsular wall (q.v.) are sometimes used synonymously for the organic wall surrounding radiolarian or clathruline heliozoan bodies, but it is preferable to separate them based on morphological differences between the wall of radiolaria (capsular wall) and the more loosely organized structure of heliozoa (capsule). A special type of extrusive organelle found in myxozoa is termed "polar capsule". Some bacteria and yeast are surrounded by a protective organic wall, also known as a "capsule". Case, see Lorica. Cell coat, see Glycocalyx.

Cell membrane, see Plasma membrane.

Cell wall. A mostly rigid, often multilayered structure consisting of discrete microfibrillar polysaccharides embedded in an amorphous matrix composed of polysaccharides, lipids, and proteins which together comprise an outermost layer of the cell proper. Architectural complexity and composition may vary among taxonomic groups, stages of cell development and environmental conditions. The cell wall confers shape to the cell and constrains the turgid protoplast. In addition, the cell wall (1) influences the cellular uptake of water and other molecules thereby regulating intracellular ionic and solute concentrations, (2) passively regulates secretions, (3) contains enzymes involved in the conversions of nutrients into metabolizable forms prior to entry into the cytoplasm, (4) serves as a store for carbon reserves, and (5) is involved in cell-cell interactions/ recognitions. See also, Cyst wall, Frustule, Spore wall, Wall fimbriae.

Cellular stalk, see Stalk.

Chitinous threads. Stiff, elongate fibrils of p-chitin (sometimes called chitan) that emanate from pores in the silica valves of some centric diatoms, and may extend a considerable distance from the cell surface. Chitinous threads (chitin fibrils) in diatoms originate from specialized invaginations of the plasma membrane that appear involved in fibril formation. One or occasionally two fibrils emit from a single pore, and appear to be replaced if dislodged from the surface. In some species chitin fibrils may be involved in colony formation. Chitin is also a major component of cell walls (q.v.) in many protists (especially fungal protists) and also occurs in loricae, thecae, and cyst walls (q.v.) of some algal and protozoan protists (chrysophytes, choanoflagellates, ciliates).

Ciliary membrane, see Plasma membrane.

Coccolith. Calcareous scale (q.v.) or plate-like particle deposited on the surface membrane of some prymnesiophytes (the coccolithophorids), varying in complexity and surface decoration according to species. Coccoliths are synthesized within cytoplasmic vesicles and deposited by exocytosis on the surface of the cell in an overlapping arrangement forming a coccosphere. Between 10 and 100 coccoliths comprise the coccosphere, which may range in size from 5 to 50 microns. The function of the coccosphere is not fully understood. Under certain growth conditions coccoliths are not secreted and the cells are naked. Coccoliths with little modification of the calcite crystal form are termed holococcoliths and are apparently developed at the surface of organic scales. Heterococcoliths are composed of identifiable component elements and are produced within the cell. A special terminology has been developed for various subtypes of holococcoliths (e.g., calyptroliths, crystalloliths) and heterococcoliths (e.g., cricoliths, placoliths, etc.).

Cortex. The outer portion or layer(s) of a protist cell, including the plasma membrane (q.v.) but excluding secreted nonliving structures that may lie outside that membrane. In ciliates, the cortex exhibits considerable thickness, like the ectoplasm (q.v.) of many rhizopod amoebae, and contains the pellicular alveoli (q.v.) and the epiplasm (q.v.) as well as infraciliary organelles, notably the kinetosomes and their associated microfibrillar and microtubular structures. It also bears the cilia. In recent literature, the term "cortex" is used almost exclusively in reference to ciliates. See also, Cuticle, Pellicle.

Costa. Siliceous rib made up of costal strips which join and make up a spicular basket or basket-like lorica (q.v.) in members of the choanoflagellate family Acanthoecidae. Term also used for an elongated, solid thickening of the diatom valves (q.v.), the "attachment band" connected at both ends to coiled filaments that confer elasticity to the cortex (q.v.) in acantharian actinopods, and a ridge or rib serving as an external ornament on the calcareous test (q.v.) of certain foraminifers, running parallel to the length or axis of the test. Within a lorica of choanoflagellates, costae are usually positioned in a plane parallel or perpendicular to the long axis of the protoplast and are called longitudinal and transverse costae, respectively. Some species may have helically or more irregularly arranged costae. For many taxa the number and organization of costae are used taxonomically. In addition to the above usage, the term "costa" is also used in certain parasitic flagellates (trichomonads) for a structure which is strictly speaking not a cell surface structure, namely a slender, rib-like structure or rod (a modified flagellar root, but not microtubular), which subtends the undulating membrane and which is often highly contractile and used in locomotion of the entire organism. The term "costa" is also widely used to describe diverse non-homologous rib-like structures in non-protist organisms, e.g., the ribs of vertebrates, the anterior vein on an insect wing, or the single midrib of a plant leaf, frond, or moss thallus.

Cuticle. In the broadest sense, a thin, cellular or non-cellular hydrophobic layer deposited on the outermost surface of the cell wall (q.v.). In euglenoid flagellates this term is sometimes used synonymously with pellicle (q.v.). In some protists also used as a synonym of cortex (q.v.). The terms "cuticle", "cutis", and "pellis" are synonyms. The cuticle functions as an effective barrier to the movement of water, leaching of nutrients, and adds protection from injuries due to wind and physical abrasion. It may be present as either a non-cellular waxy layer on hyphae and spores or as a cellular layer of compressed hyphae on fruiting bodies. Multiple layers may be identified in the latter where the uppermost is referred to as the epicuticle and the next below as the subcuticle. Third and fourth layers, if present beneath the subcuticle, are the hypodermial and the subhypodermial layer, respectively.

Cutis, see Cuticle.

 Cyst wall. The cell wall (q.v.) of a cyst (a nonmotile, often dehydrated, resistant, inactive, dormant stage in the life cycle of many protists). Cyst walls or cyst membranes, sometimes multiple in number and quite complex, may bear sculpturing or protrusions of a striking nature, and sometimes may be equipped with an emergence pore (plugged or unplugged). Cysts are generally considered to serve in protection, propagation, and/or dispersal of the species. Many kinds of cysts have been described for protozoan protists alone, and very likely they are not all homologous structures. They are characteristic of numerous free-living as well as parasitic groups of protists and are useful in taxonomy. Planktonic foraminifera (e.g., Globigerinita glutinatd) are occasionally enclosed by a thickened protective wall, surrounding the shell (q.v.), forming a "feeding cyst" containing undigested remains of prey such as diatom frustules. Some cysts contain a cyst plug, a non-living substance as a protective closure within the opening. The term cyst (cyst wall) is sometimes used synonymously with spore [spore wall (q.v.)]. See also, Pellicle.

Cytokalymma. A thin cytoplasmic sheath surrounding the siliceous endoskeleton (q.v.) of some actinopods (e.g., radiolaria) that supplies the silicate during skeletal deposition; the geometry of its internal space determines the shape of the skeleton. The cytokalymma is a dynamic structure that enlarges with the growing skeleton. By cytoplasmic streaming and changes in form, it determines the ontogenetic sequence of stages during skeletal growth.

Diatotepum. A distinct, continuous organic layer found between the siliceous frustule (q.v.) and the plasma membrane (q.v.) in diatoms. Previously referred to as the "pektin" or "diatopectin" layer, this terminology was confusing due to the possible chemical interpretations. The diatotepum stains for acidic polysaccharide and has been described for a number of diatoms, mostly centric. The diatotepum is considered a distinct layer (= "diatotepic layer") in addition to the organic coat (= organic casing). The latter envelopes the surface of all siliceous components of the frustule.

Discharge plug . Gelatinous, highly refractive material secreted between the plasma membrane (q.v.) and sporangial wall at early stages of spore formation in chytrids. Found among diverse chytrids including those with operculate and inoperculate zoospore discharge.

Discharge vesicle. Extracellular material produced during zoosporogenesis in fungal and algal protists, which at spore or protoplast discharge, evaginates into an external sac, constraining spores until they rupture the vesicle. Involved in zoospore discharge of many algae and oomycetes such as Pythium and chytridiomycetes such as Chytriomyces. Also appropriately applied to the polaroplast of Microsporidia. See also, Gun cell.

Ectoplasm. Outer layer of the cytoplasm of a cell, bounded by the cell or plasma membrane (q.v.). In many rhizopod amoebae, it may be of considerable thickness, clear, non-granular, and more viscous than the underlying endoplasm. In ciliates, the special name "cortex" (q.v.) is often used for essentially this layer of protoplasm. See also, Ectoplasmic net, Pellicle.

Ectoplasmic net. Slime filaments interconnected into a fine network generated from a specialized mem­brane aggregate, the sagenogen. The sagenogen was first termed the bothrosome in the labyrinthulids (meaning pit body), and later the sagenogenetosome (meaning net-producing body) in the thraustochytrids. Sagenogen is a modification of the later term. In labyrinthulids, spindle-cells glide within the net; and in thraustochytrids, the net extends from the sporangium as rhizoidal-like structures. In both groups, the net is actually continuous with the plasma membrane (q.v.) which protrudes from a pore within the layer of cell-covering scales (q.v.). At the light microscopy level, the ectoplasmic net has the appearance of extracellular material, but electron microscopy has revealed that in strict terms it is not, but rather is simply an elaboration of the plasma membrane.

Elaters, see Capillitium.

Endooperculum . Wall-like layer deposited between the plasma membrane (q.v.) and spore discharge area. In chytrids, it is typically secreted between the plasma membrane and discharge plug material just prior to zoosporogenesis. Distinguished from an exooperculum [ = "operculum" (q.v.)] because it is distinct from the original sporangial wall and is an additional layer deposited late in sporangial development.

Endoskeleton, see Skeleton.

Envelope. A general term in protistology used variously for such structures as plasma membranes, cystic membranes, pellicles, coats, walls, sheaths and gelatinous coverings, and even shells or loricae. The range of cell coverings loosely called "envelopes" precludes their consideration as homologous structures. See also, Lorica.

Epiplasm. A peripheral, cytoplasmic layer of varying thickness directly underlying the innermost cell mem­brane; thus, in ciliates, it is positioned just beneath the pellicular alveoli (q.v.). The epiplasm is part of the cortex (q.v.) of ciliates and in some species can be subdivided into a discontinuous set of plates.

Exooperculum, see Operculum.

Exoskeleton, see Skeleton.

Extracellular matrix. Mucilaginous glycoproteins external to the plasma membrane (q.v.) or to the compact, microfibrillar cell wall (q.v.), characteristically appearing flocculent, loosely fibrillar, finely granular, or amorphous. It may totally surround a cell or be localized to specific regions at the cell surface. Certain zones may not be preserved or visualized with standard conventional fixation for electron microscopy, but may be revealed using cationic dyes such as ruthenium red or cryofixation techniques. This is a broadly descriptive term which encompasses different functional categories. "Cell coat" and "glycocalyx" (q.v.) refer to extracellular matrix adjacent to the plasma membrane. "Adhesion pad", "appressorium ring", "hyphal sheath", and "spore mucilage" (q.v.) describe extracellular matrix adjacent to the cell wall. Reported functions of this material include adhesion, collection of functional degradative enzymes and cell protection either with prevention of cell desiccation or binding of toxic compounds. See Conclusions and recommendations; see also, Holdfast adhesion, Sheath, Slime track.

 Exudates, see Spore mucilage.

 Frustule. All siliceous components of the vegetative cell wall (q.v.) of diatoms, comprising two valves (q.v.), the epivalve and hypovalve, and the girdle (or cingulum). The epivalve plus epicingulum comprises the epitheca, which is that component of the frustule inherited from the parent cell during division. The hypovalve plus hypocingulum constitutes the hypotheca, or that component of the frustule deposited by daughter cells following division. The frustule plus any additional organic component(s) (see Diatotepum), regardless of the composition, are together referred to as the diatom cell wall. It has been reported that silicification of the frustule may occur on an organic matrix (template?), though this has not been conclusively demonstrated. Frustule components, particularly the valves, are normally adorned with a vast range of morphological fea­tures defined by an equally vast and precise terminology. See also, Auxospore wall.

Fuzzy coat: see Glycocalyx.

Glycocalyx. An organic coat, covering, sheath (q.v.) or extracellular matrix (q.v.) extending from the outer surface of the plasma membrane (q.v.); e.g., the organic layer coating the surface membrane of many "naked" rhizopod amoebae, the mucilaginous cell coat covering the plasma membrane in some "naked" algal protists (e.g., the green alga Dunaliella), the cell coat on zoospores of some oomycetes (e.g., Phytophthora, Lagenidiuni) and chytridiomycetes (e.g., Polyphagus, Chytriomyces), or the surface covering of other protists ranging from trypanosomes to ciliates. The surface coat, among other surface membrane components, confers antigenic cellular specificity. Variations among "naked" amoebae include surface coats that: (1) are amorphous and thin or thick, (2) consist of various configurations that are not resolvable into separate structures (fuzzy, with a suggestion of an organized pattern, or containing hexagonal or tubular elements), or (3) contain more discrete structures, varying in geometry, arranged in a definite pattern, and not separable intact from the surface membrane (see Glycostyle).

Glycostyle. Complex, flexible, discrete structures at­tached to the surface of cells (especially "naked" amoebae), each separate from its neighbors, arranged in a definite pattern, and not removable intact from the cell surface. In "naked" amoebae, the glycostyles vary among genera (e.g., pentagonally symmetrical in Vannella, and hexagonal in Vexilliferd). Glycostyles are distinguished from other discrete surface structures such as scales, plates, and spicules (q.v.), by being flexible, sometimes erect, projections from the cell surface.

Gun cell. Developing from an encysting zoospore of the oomycete Haptoglossa, this apparatus consists of a cell with: (1) an extracellular plug and projectile located in an invaginated chamber of the cell surface and (2) an intracellular complex system of vacuoles. These components are used to forcibly discharge the missile-shaped projectile into rotifer or nematode hosts which collide with the cell, injecting infective sporidia through the host cuticle and into the host body where the fungal-protist grows into cylindrical thalli. This mechanism of host infection is similar to one found among some members of the plasmodio-phoromycetes infecting plants (see Stachel), but differs in its explosive trigger and discharge of the structure used to pierce the host surface. The discharge vesicle (q.v.) of the Microsporidia can be considered a type of gun cell.

Holdfast adhesion. Extracellular matrix (q.v.), mucilaginous secretion, cementing one end of a filament, typically via a morphologically distinct holdfast cell, to a substrate. Found with single algal filaments, particularly in shallow aquatic habitats. In actual use the term holdfast sometimes applies to a cell, to extracellular matrix, or to both a cell and secreted adhesive matrix. This term is also used among fungi such as with trichomycetes inhabiting guts of arthropods as commensals. The sessile state of the ciliate Stentor and many other sessile protists also produces an adhesive attachment at the posterior end. See also, Adhesions, Adhesion pad, Holdfast organelle, Stalk.

Holdfast organelle. Term broadly used in protozoology in reference to any structure(s) by which a given organism can affix or attach, temporarily or permanently, to some living or inanimate substrate. A wide range of organelles, often specialized and involving secreted or otherwise formed non-living and/or living structures, are implicated: e.g., cilia (individual or in thigmotactic fields), flagella, pseudopodia, tentacles, tails, suckers, hooks, spines, epimerites, sucking discs, threads, filaments, stalks of diverse kinds and loricae (q.v.). As widely used by protozoologists, these diverse holdfast or adhesive organelles go beyond the rather restricted concepts of adhesions (q.v.) and hold­fast adhesion (q.v.) as defined above.

House, see Lorica.

Hyphal sheath, see Sheath.

Hypothallus. A thin clear crust at the base of fruiting bodies of some Mycetozoa. This structure is continuous with the acellular stalk (see Stalk) and appears as a disc. In protostelids this structure is also called a "basal disc" (q.v.) and in certain slime molds the hypothallus may be a continuous structure interconnecting the bases of multiple fruiting bodies. In coralline red algae the term "hypothallus" (= "hypothallium") is used for the lower part of the thallus composed of large cells.

Idiosome. An inorganic surface particle in the wall of testate amoebae or other biomineralizing protists, secreted within cytoplasmic vesicles and deposited on the surface of the newly formed cell during binary fission. Idiosomes, secreted by the cell, are distinguished from particles collected from the environment and cemented into the wall [= xenosomes (q.v.)]. The shape and chemical composition of idiosomes vary among species of testate amoebae and include pearl-shaped, globose, ovate, peg-shaped, or plate-like structures that may be either siliceous or calcareous.

Inner membrane complex, see Alveoli.

 Keel. Any ridge or flange-like extracellular thickening resembling the longitudinal plate or timber (keel) on the bottom of the hull of a ship. In foraminifera, some species are reinforced by a thickened ridge along one edge of the shell; in some pennate diatoms, it occurs on the valves (q.v.); in euglenoid flagellates, the term "keel" is used to describe prominent pellicular ridges.

Lacuna, see Alveoli.

Lorica. A structure surrounding or enveloping the body of some ciliates and flagellates; it fits loosely over the body proper of the organism, with a wide opening at one, occasionally both end(s), and is sometimes attached to a substrate at the other pole (see Holdfast organelle). In tintinnid oligotrichs and flagellated protists it is often carried about by the organism freely swimming in the water column. Its composition may be of a calcareous, chitinous, pseudochitinous, proteinaceous, siliceous, or tectinous nature, or it may be mineralized by iron and manganese compounds, or it may be made up of sand grains, coccoliths, diatom frustules, or simply debris held together by a secreted "glue". Broadly speaking, the following terms are sometimes synonymized with "lorica": basket, case, envelope, house, sheath, shell, test, theca, tube, valve. Loricae are secreted or assembled by their occupants; when not solitary, they may appear in an arboroid "colony" arrangement. When an occupant has undergone cellular division during asexual reproduction in the lorica, one of the filial organisms often departs to secrete its own house elsewhere, the other remaining, often attached, in the parental abode. Peritrichs, folliculinids, and tintinnines are the major ciliate taxa in which loricae are to be found. As for flagellated protists, loricae occur in some choanoflagellates (acanthoecids), some (fossil) ebriids, euglenoid flagellates (e.g., Trachelomonas), chlorophytes (e.g., Phacotus) and heterokonts (e.g., Bicosoeca, Dinobryon, Epipyxis). Epipyxis produces a lorica composed of imbricate plates or scales (q.v.) of fibrillar construction. The loricae of acanthoecid choanoflagellates are uniquely basket-like, composed of costal strips (see Costa, Skeleton). A fine membranous or fibrillar investment ("lorica membrane") lines part of these basketlike loricae and an extension of this attaches to the protoplast, holding it firmly in position within the lorica. The lorica of fossil ebriids is an inflated chamber-like structure which generally develops at the anterior end of the skeleton (q.v.).

Mucous pad, see Adhesion pad. Nectar, see Spore mucilage.

Operculum. Literally, lid or bowl-shaped flap. Term used for diverse protist structures including (1) the "plug" of an emergence pore of a cyst or spore, (2) the whole or partial covering of certain peritrich loricae (q.v.), (3) the so-called "epistomial disc" (an anterior part of the body of certain other peritrich ciliates), or (4) the circumscissile bowl-shaped flap dissolved from a fungal sporangial wall allowing spore discharge. In fungi, "operculum" is sometimes synonymized with "exooperculum". Operculate (= exooperculate) discharge occurs on sporangia releasing zoospores among some chytrids. This term is also used among fungi describing asci-releasing ascospores.

Outer rodlet layer. Distinct zones of rods scattered in spore walls (q.v.) of ascospores and conidia; best visualized with freeze-etch techniques. In hydrophobic conidia the structure ini­tiating interface with host insect cuticle of invertebrate fungal pathogens.

Pedicel, see Stalk.

Peduncle, see Stalk.

Pellicle. (1) Proteinaceous plate or plates located just underneath the plasma membrane (q.v.) in euglenoid flagellates. The individual plates (pellicular strips) may be capable of sliding along each other as in Euglena, or the plates may be firmly attached to each other as in Phacus. (2) In ciliates, "pellicle" is used to describe the outer part of the cell, comprising plasma membrane, alveoli, and the epiplasm (q.v.), but it is only the outer portion of the entire ciliate cortex (q.v.). (3) In dinoflagellates, it is used as a term for the "wall" of temporary cysts developing after the theca (q.v.) is shed by ecdysis. This type of pellicle comprises the fused inner amphiesmal vesicle (q.v.) membranes, an internal probably cellulosic layer and an external thin cellulosic (sometimes sporopollenin-containing) layer which originates from material present in the amphiesmal vesicles. (4) In various other protists (e.g., apicomplexans, opalinids, acantharians, scattered other taxa), "pellicle" has been used, rather loosely, for the plasma membrane plus any thickened material just below it, reminiscent of the terms "cortex" (q.v.) and "ectoplasm" (q.v.). (5) The term "pellicle" has also been used to denote the skin-like aggregations of zygotes developing after clumping of gametes of opposite mating types in chlamydomonadalean flagellates. The pellicle of some euglenoid flagellates (e.g., Euglena spirogyra) is decorated by ferric hydroxide "warts". The term "pseudopellicle" has sometimes been used for euglenoid flagellates to describe the plasma membrane and the underlying proteinaceous plates. See Conclusions and recommendations; see also, Cuticle, Periplast.

Pellis, see Cuticle.

Peridium. A covering enclosing spores within fruiting bodies. In Mycetozoa the peridium originates from material secreted as plasmodia differentiate into fruiting bodies. In zygomycetes the peridium is the original sporangial wall enclosing spores after they are cleaved from a multinucleate sporangium and produce spore walls (q.v.). With fruitifications of higher fungi, the term is not restricted to extracellular material. Among rust fungi peridium pertains to the cup composed of hyphae surrounding aeciospores of fruiting bodies called aecia.

Perilemma , see Plasma membrane.

Periplast. In many protists, a colloquial term for a covering on the outside of the plasma membrane (q.v.), e.g., "a scaly periplast". In cryptomonad flagellates, a trilaminate structure consisting of proteinaceous inner and surface components that are closely associated with (i.e., "sandwich") the plasma membrane. In euglenoid flagellates, sometimes used as a synonym of "pellicle" (q.v.). In cryptomonad flagellates, the periplast covers the entire surface except for a subapical vestibulum from which the flagella emerge. Components are often composed of highly organized plate areas that correspond in size and position on either side of the plasma membrane. There is great variation on this simple theme; the inner component may consist of a single sheet or discrete plates that have variable relationships with the plasma membrane. The surface component, the details of which can only be viewed following freeze-fracture/etch, displays an even greater variation in structure; consisting of discrete plates or scales or fibrils or mucilage and combinations of any or all of these structures. See Conclusions and recommendations.

Perizonium , see Auxospore wall.

Plaque . (1) An organic plate-like structure cemented among the inorganic particles in the wall of testate amoebae; discoidal, with complex areolate or reticulated pore pattern in the central space. (2) In ciliates, a rectangular arrangement of intramembranous ultramicroscopic particles in the ciliary membrane covering a cilium, at the proximal end of the cilium. The plaques are secreted within Golgi-derived vesicles in the cytoplasm of testate amoebae, and are deposited by exocytosis on the surface of the newly formed cell during binary fission. Organic cement, secreted by other Golgi-derived vesicles, forms the matrix of the wall containing the plaques and test particles. Note that the term "adhesion plaque" [= "adhesion pad" (q.v.)] has a different meaning. "Plaque" is also used in bacteriology to describe a clear zone in a bacterial lawn on a culture plate. See also, Pore plate.

Plasma membrane. With rare exceptions, the outermost living membrane of all protistan cells; therefore, the homologue of the universal limiting unit "cell membrane" of the cytological literature, possessing the same ultrastructure and general functions. While "cell envelope", "cell membrane", "cytoplasmic membrane", and "plasmalemma" are often used as synonyms of "plasma membrane", certain other terms may relate to structures that either sometimes include the cell membrane, e.g., the cortex (q.v.) and, in ciliates, but not in euglenoid flagellates, the pellicle (q.v.), or are positionally in juxtaposition to it, e.g., the perilemma, an additional living membrane that lies just outside the plasma membrane (ciliary membrane), in some oligotrich ciliates and certain stichotrichous hypotrichs, and the alveoli (q.v.), with their own unit membrane, lying just inside (under) it. Although we are not including motile appendages in our treatment, some remarks are made on clarifying the use of the term "membrane" and "membranelle". The plasma membrane (often called the "ciliary membrane" by ciliatologists) of the cell body also encloses, without a break, the central axoneme of the protruding flagellum or cilium; whether such structures are present individually or, as in many ciliates, in some compound form. The latter are sometimes called a membrane or membranelle (e.g., the paroral or undulating membrane and the adoral zone of membranelles or polykineties of the oral or buccal area and the cirri on the ventral somatic surface of hypotrichs). See also, Glycocalyx.

Plate. A relatively thin, flattened structure depos­ited on the surface of cells, usually inorganic (calcareous or siliceous) or organic as in the case of thecal plates in dinoflagellates; varying in geometry, circular, elliptical, quadrangular, or irregular in outline. Plates are secreted within cytoplasmic vesicles and deposited by exocytosis on the cell surface in some amoebae and centrohelidian heliozoa (or cysts in actinophrydian heliozoa), or enclosed in amphiesmal vesicles (q.v.) in dinoflagellates. A layer of skeletal plates is secreted within alveoli (q.v.) of the ciliate Euplotes. The plates may be planar to slightly concave, concavoconvex, or biconvex; perimeters variable, round to polygonal. Plates can be considered a form of scale (q.v.) or a synonym, but in general a plate is much thinner than broad, whereas a scale may be much more elaborate in three-dimensions. See also, Lorica, Periplast, Pore plate.

Pore plate. An organic or mineralized septum containing pores. In foraminifera, some pores in the shell contain septa, spanning the opening usually at a position below the rim, and penetrated by micropores that often contain thin, cytoplasmic projections. A fine silicified plate with pores that span the areola of the frustule (q.v.) of many diatoms. An organic, sometimes partially silicified, areolate plate or plaque (q.v.) is deposited in the test (q.v.) of some testate amoebae.

Properizonium, see Auxospore wall.

Pseudopellicle, see Pellicle.

Pustule. Blister-like, papillate, or knob-like projection on the surface of a cell enclosure, or eruptive spot or spore mass in fungi. Pustules occur as calcified projections on the outer surface of foraminiferal shells, or as ornamentations on the surfaces of frustules and thecae (q.v.) of some algae.

Sagenogen, see Ectoplasmic net.

Scale. An organic or inorganic cell-surface element of variable geometry, distributed individually or arranged in a pattern sometimes forming an envelope around the cell. The scale may be thin and plate-like (solid and ornamented or perforated with varying patterns), composed of meshwork or latticed structures, or complex, three-dimensional shapes forming basket-shaped, boat-shaped, crown-shaped, tower-like, or spine-bearing structures (as in some flagellates, amoebae and heliozoa). Surface scales are secreted by a wide range of protist taxa spanning diverse algae, flagellates, sarcodinians, ciliates, labyrinthulids, thraustochytrids and protostelids. Bristles, coccoliths, and plates (q.v.) may be considered special forms of surface scales. The fine structural features of scales, as well as the position and manner of their formation and deployment, are important taxonomic characteristics. See also, Auxospore wall, Lorica.

Sheath. A general term for some sort of outer covering or extracellular matrix (q.v.) of a protist cell or of an extension of such a cell: e.g., the gelatinous investment outside the cell wall of some coccoid and filamentous algae; the non-living outer covering, or annulus, of the stalk (q.v.) of many peritrich ciliates; the material surrounding a germ tube of di­rectly germinating oomycete cystospores. Sheaths also commonly occur in cyanobacteria. The sheath in most organisms is composed of mucopolysaccharides or glycoproteins. The degree of development of a sheath is sometimes associated with the type of the substrate the cell contacts. "Hyphal sheath" refers to extracellular material along hyphae of oomycetes as well as "higher" fungi. Hyphal sheaths are particularly prominent with aquatic hyphomycetes. The degree of development of a sheath is sometimes associated with the type of substrate the hypha contacts. See also, Adhesion pad, Envelope, Glycocalyx, Lorica, Slime track.

Shell. A shell is a protective inorganic or organic enclosure, deposited directly by the organism or built-up by cementing foreign particles into a wall matrix. Its wall is usually solid and smooth or variously ornamented. One or more openings (apertures, pores, etc.) provide access to the surrounding environment. Also used for the cellular enclosure known as a "spore membrane" enclosing myxosporidian spores; the myxosporidian shell is composed of two to six valves (q.v.) or sections with thickened edges and often bearing sculpturing, markings, or processes on the outer surface. The shell geometry is often used as a taxonomic indicator. "Shell" and "test" (q.v.) are some­times used synonymously, and a clear distinction is not easily maintained; however, in general, a shell may be considered to be a rather thick, wall-like or framework enclosure with one or more major openings. See also, Envelope, Lorica.

Skeleton. A hardened non-living, protective or supportive structure, enclosed by, or attached to, cytoplasmic structures. In a strict usage, it includes only non-living frameworks and spine-bearing structures that form anchorage or support for cytoplasm. This excludes smooth-surfaced, solid-walled shells, tests, or loricate structures lacking supportive spines or open frameworks. An endoskeleton is partially or completely enclosed by cytoplasm; an exoskeleton lies totally outside the cytoplasm, though cytoplasmic structures may be attached to it. Few protists have a true exoskeleton, since most of the skeletal frameworks are at least partially surrounded by cytoplasm at some time (e.g., in diatoms). The elaborate siliceous skeletal frameworks of radiolaria and the spine-bearing shells of planktonic foraminifera may be considered endoskeletons since they are partially surrounded by, and form a supportive framework for, cytoplasm. External skeletons occur in silicoflagellates, but evidence indicates that they are formed internally and then extruded by the cell (detailed ultrastructural studies on the origin of the skeleton are missing). The skeletal elements of the basket-like lorica (q.v.) of choanoflagellates are also produced intracellularly and extruded to the cell surface; when a full complement has been produced, they are rapidly assembled to produce a basket (q.v.).

Slime sheath, see Slime track.

Slime track. Extracellular matrix (q.v.) sloughed off plasmodia of slime molds and most amoebae, particularly amoebae as pseudoplasmodia of cellular slime molds migrate prior to culmination and differentiation of sorocarps. This material is most prominent in cellular slime molds that do not lay out a horizontal stalk during migration. The term "slime sheath" is most widely used to describe the continuous, extracellular material that remains around the pseudoplasmodia.

Somatonemes. Surface, hair-like structures on the soma (body) of some flagellates such as proteromonads. The somatonemes are synthesized in the endoplasmic reticulum and migrate to the Golgi vesicles before they are discharged on the cell surface. There they appear to be linked to cortical microtubules by anchoring proteins which are part of the plasma membrane (q.v.).

Spicule. A rod-like, spindle-shaped, stellate, or variously curved and ornamented structure, usually siliceous or calcareous, with blunt or tapered tips deposited individually on the cell surface or distributed throughout the peripheral cytoplasm. The term spicule is occasionally used to mean a tapered rod-like extension attached to a skeletal framework as in radiolaria, but preferentially the term spicule is used for unattached elements. The geometry of the scattered spicules is a taxonomic characteristic of some actinopod species. In some solitary and colonial radiolaria, the spicules may be lunate, tri-radiate to hexa-radiate, s-shaped, or variously curved, needle-like elements scattered within the gelatinous coat surrounding the cell. Among other examples, curved spicules occur in centrohelidian heliozoa (produced within specialized organelles called "spicule-generating organelles"), and calcareous spicules are secreted by the marine amoeboid testacean Trichosphaerium spp. Endoskeletal siliceous spicules occur in some dinoflagellates (e.g., in the genus Plectodiniuni). The framework of thin, siliceous rods surrounding some choanoflagellates (e.g., Diaphanoeca spp.) is known as a spicular basket (see Costa). A wide variety of siliceous spicules are also secreted by sponges and form supportive structures for the soft body.

Spine. A non-living (e.g., organic, calcareous or siliceous), rod-like or tapered, sometimes ornamented, elongate structure attached to a scale, shell, wall, or skeletal framework. Spines can serve as supportive structures for cytoplasm and as protection. For example, in radiolaria and planktonic foraminifera the spines are attachment sites for feeding rhi/opodia and provide a mechanical advantage during capture of large prey. The cross-section may be round, elliptical, triradiate, or of complex design and the surface may be smooth, supplied with short spines or embellished with a variety of ornamentations. Prominent spines occur on frustules (q.v.) of some diatoms, on scales (q.v.) of some chrysophytes and synurophytes, and on dinoflagellate cysts. In the synurophyte genus Mallomonas, stiff spines on the scales should not be confused with movable bristles (q.v.), which are elongate structures developed intracellularly separate from scales, joining together with scales only after their release on the cell surface.

Spore mucilage. Extracellular matrix (q.v.) commonly believed to be rich in carbohydrates, preventing spore desiccation or attracting and attaching to animal vectors. Spores released from generative structures typically cluster in the mucilage as a globular mass. "Exudates" and "nectar" are synonyms of "spore mucilage". Examples include spermatia of black-stem rust of wheat fungus on barberrry host, conidia of Dutch elm disease fungus, conidia of chestnut blight fungus, conidia of ergot fungus, oidia of inky cap mushroom, and ascospores of yeast Dipodascopsis uninucleatus.

Spore tip mucilage, see Adhesion pad.

Spore wall. The cell wall (q.v.) of a spore (a microscopic propagule capable of giving rise to a new individual(s) or transferring genetic information). Surface features, architectural complexity and chemical composition of the spore wall vary among taxonomic groups. The fungal spore wall serves as a protective barrier from environmental stresses (i.e., cold, heat, desiccation, radiation) and microorganism attack and may aid in spore dispersal. The surfaces of spores may be smooth or ornamented. Examples of surface ornamentation include appendages (conidia of Pestalotia macrotrichd), warts (ascospores of Genea hispidula), spines (uredospores of Puccinia graminis), reticulations (ascospores of Tuber rufum), verrucae (aeciospores of Cronartium quercuum), and ridges (ascospores of Neurospora crassa). Spore surfaces may also have scars, pegs, tubes (all marking the points of attachment), or germ pores. Many spores have a fine ornamentation that is detected only with high resolution electron microscopy (see Outer rodlet layer). The spore wall is typically composed of more than one wall layer and may have as many as five or more distinct layers when examined with transmission electron microscopy. Layer terminology varies between authors. The most commonly used terms, starting from the inner wall layer, are: (1) endosporium (= endospore), (2) episporium (= epispore), (3) exosporium (= exospore, epitunica, trachytectum), (4) perisporium (= mucostratum, myxosporium), and (5) ectosporium. Some parasitic protozoa (e.g., apicomplexans, microsporidians, and myxosporeans) produce a small dispersal, infective stage known as a "spore". This, however, is not strictly equivalent to the environmentally resistant or quiescent dispersal stages cited for fungi and other protists. The "spores" of these parasitic protozoa are sometimes en­closed by valves (q.v.) with ridges or thickenings at the point of contact (sutures), and may have additional surface features such as a mucus coat, filamentous fleece, elongated caudal appendage, or warts that aid in their dispersal. The quiescent stages of free-living protozoan protists are known as cysts rather than spores. See also, Auxospore wall, Cyst wall.

 Stachel. An osmiophilic knife-shaped structure which develops outside the cell in a cavity, the tubular invagination of the plasma membrane (q.v.) termed "Rohr", in encysted zoospores of plasmodiophoromycetes. The Stachel is surrounded by adhesive material and after contact with a plant host, the Stachel pierces the cell wall, allowing entry of the parasite protoplast. Although this structure might be considered an example of an extrusive organelle, it is assembled external to the plasma membrane and is not an intracellular organelle.

Stalk. An elongate structure specifically formed (1) to attach an organism to a living or non-living substrate or (2) to raise spores or group of spores (e.g., sporangia) above a substrate or host, a strategy which increases spore dispersal potential by wind or animal vectors in terrestrial habitats or by turbulence in aquatic environment. Broadly speaking, the following terms are sometimes synonymized with "stalk": pedicel, peduncle, and stipe. These are general terms widely used for both non-living and living structures. The various kinds of non-living stalks may have different origins. The acellular stalk, as found among some myxogastria, protostelids, and chytrids, arises as a mound or tube from secretion of extracellular material. In some protostelids and chytrids the acellular stalk is hollow, and in protostelids the microfibrillar stalk tube is covered with a sheath (q.v.). Algae (e.g., diatoms) often attach to substrates with polysaccharide acellular stalks. Living stalks can be merely extensions of the body in many algal and protozoan protists. Cellular stalks of dictyostelids form as cells aggregate and raise the sorocarp above the substrate surface and die at culmination of fruiting body formation, leaving their cell walls as a dictyoid, non-living stalk. The cellular stalks of dictyostelids may be homologous to those of the acrasids which are composed of living cells. Multicellular stalks (stipes) also attach some algae (e.g., benthic, macroscopic brown algae) to substrates. In ciliates (e.g., peritrichs and suctoria), stalks commonly arise by extension of (living parts of) the body of the organism or by secretion of specific non-living materials or by some combination of these means. The stalk is sometimes produced from a specialized region known as a scapula in suctorian and peritrich ciliates. See also, Adhesions, Adhesion pad, Basal disc, Holdfast adhesion, Holdfast organelle, Hypothallus, Sheath.

Stalk tube, see Stalk.

Stipe, see Stalk.

Surface coat, see Glycocalyx.

Test. A hardened cell covering, forming a partial or complete enclosure, typically secreted by the organism or built up of particles gathered from the environment, forming a protective barrier around the cell; often used as a general descriptive term for a wide variety of hard-coverings, loricae, shells, thecae, or valves (q.v.). The term "test" lacks much specificity in its typical usages being applied to diverse enclosures including those of diatoms, testate amoebae, and some monothalamic foraminifera. In general, a test may be defined more restrictively as a relatively thin-walled enclosure that almost fully surrounds the cell, with one or only a few major openings; this is distinguished from a shell that is thick-walled and may have many pores and openings, or a lorica that more loosely encloses the cell and has few to many openings.

Theca. Term used in various connections throughout protist taxa to describe a wall-like structure. The theca of dinoflagellates is composed of thecal plates, each lodged in a vesicle [amphiesmal vesicle (q.v.)] located inside the plasma membrane. In diatom frustules (q.v.), the valve (q.v.) together with the girdle elements (cingulum) associated with it is called the "theca". In the prasinophytes Tetraselmis and Scherffelia, the theca is an extracellular more or less close-fitting membrane surrounding the cell. It is composed of fused organic scales. In the choanoflagellates, members of the Salpingoecidae are surrounded by a theca, an extracellular usually close-fitting membrane, which may be extended into an anterior collar and posterior stalk. Epitheca is the anterior part of the theca in dinoflagellates (the area of the cell in front of the cingulum or girdle), the hypotheca is the posterior part of the theca. In diatoms, the older valve and its associated elements is called "epitheca", the hypotheca is the newer part of the theca. Testate amoebae, enclosed in an organic or mineralized shell (q.v.), are sometimes referred to as thecate amoebae. See Lorica, Test.

 Valve. Major section of a frustule, theca, test, or shell (q.v.) that is more or less recognizable as separate structure. Valves occur, for example, in diatoms and myxosporidia. In the so-called shell valves of the living cellular "spore membrane" of myxosporidia, there may be from two to six valves, with suture planes between them and, often, special sculpturing on their outer surfaces. See also, Lorica, Spore wall.

Wall fimbriae. Extensions of the cell wall (q.v.) that radiate as fibers perpendicular to the cell surface. Often found on pathogenic yeast and smuts, they are thought to have a role in determination of fungus and host interaction. Extracellular protein fibrils that appear to be serologically related to the fimbriae or fungi have also been observed on the cell surface of prymnesiophyte flagellates.

Xenosome. A particle collected from the environment and cemented into the organic matrix of a wall. Typically used for foreign mineral matter incorporated into the wall of testate amoebae or other agglutinate-wall-building protists. Xenosomes are distinguished from idiosomes. The latter are secreted de novo within cytoplasmic vesicles. The chemical composition of xenosomes, collected from the environment, depends in part on the availability of particles; however, in testate amoebae, species-specific selection, based on chemical composition of the particles, has been reported. (Although not in reference to a surface structure, the word "xenosome" has recently been introduced into the evolutionary protistological literature as a name for an intracytoplasmic or intranuclear organelle or body that contains DNA and is membrane-bounded and that is, or historically is presumed to have been, an endosymbiotic entity within a given protistan cell, having invaded or been engulfed by the host organism in the recent or distant past, subsequently becoming a more or less permanent inclusion in the host body, e.g., Kappa particles in the ciliate Paramecium and the plastids and mitochondria so common in protists belonging to many taxonomic groups.)