Asellota (Crustacea: Isopoda) Glossary

George D.F. (Buz) Wilson

This glossary contains definitions of morphological terms as provided by Wilson (1989) (with an emphasis on the asellotan superfamily Janiroidea) and Wetzer et al. (1997) (more generally to Crustacea and Isopoda). At the moment, the set up is fairly primitive, so use your browser's search function to find a particular word.

Aesthetasc. A long, tubular sensory seta having thin cuticle, found on the antennula. Aesthetascs may have a chemosensory function, because males generally have many more than females.

Ambulosome. The part of the thorax of the Munnopsidae that bears the walking legs. It consists of pereonites 1-4. Ordinarily this term is only used in this family, because other families are not modified in this way, and the posterior pereopods V-VII may be also used in walking.

Ambulosomite. A body segment of the ambulosome.

Annulus (plural annuli). A distal segment of the either the antenna or antennula, generally tubular in form.

Antenna (synonym, second antenna). The second, paired, cephalic appendage. It consists of four short, robust, proximal segments, two long, intermediate segments, and a long series of tapering annuli, called the flagellum. The third segment sometimes bears a small lateral appendage called the antennular scale that is homologous to the antennal scale seen in other Malacostraca, or the antennal exopod in other Crustacea.

Antennula (synonyms first antenna, antennule). The first paired cephalic appendage. It consists of a large basal segment, two segments of intermediate thickness, and distal annular segments of varying lengths. The most distal segments generally bear aesthetascs.

Appendix Masculina. An alternative name for a stylet-like copulatory structure on the male pleopod II. This structure is not homologous to similarly named structures found in non-isopod Eumalacostraca.

Article. A segment of any limb, but usually applied to the antennula or antenna.

Basis (plural Bases). The second segment of a thoracic limb. See pereopod.

Biarticulate. Consisting of two articles or segments.

Bifid. A structure with two distal tips, as in unequally bifid seta.

Biramous. Having two branches, a typical condition for most primitive crustacean appendages.

Brooding Female (synonym ovigerous female). An adult female with fully-extended oostegites on the coxae, forming a brood chamber that retains developing embyros. In most deep-sea samples, developing embryos are lost during sample processing, so one is uncertain whether the female was brooding embryos, or whether she released the young before sampling and had not moulted to a preparatory stage.

Carpus. The fifth segment of a thoracic limb. See pereopod.

Cephalic Dorsal Length. The length of the cephalon measured in a straight line along the dorsal midline from the posterior edge to the anterior vertex or rostrum, depending on which is present.

Cephalon. The head, or anteriormost body unit. In isopods, the cephalon bears the eyes, mouth, antennulae, antennae, and 4 pairs of mouthparts (mandibles, maxillulae, maxillae, and maxillipeds).

Chaetotaxy. The form, number, and shapes of the setae.

Circumgnathal. Around the biting or grinding surface, as in circumgnathal denticles.

Claw, Dactylar. A modified seta found on the distal segment of the walking legs that is heavily sclerotized and has a sharp tip.

Cleaning Setae. The unusual multisetulate setae found on the distal segment of the mandibular palp that are used to clean the antennae or antennulae.

Clypeus (synonym pseudorostrum). An unpaired anterior unit of the cephalon bearing the labrum medially and the mandibular fossae ventrally on its lateral margin. The articular condyles of the mandibles insert in these fossae, or ventral grooves. In some species (e.g., the Eugerda fulcimandibulata species complex, Desmosomatidae) the external margins of the fossae project anteriorly as rounded knobs.The clypeus articulates with the head across the anteroventral surface and with the labrum on its ventral margin with distinct suture lines.

Condyle, dorsal. A heavily sclerotized projection of the dorsal surface of a mandible that articulates with the cephalon and clypeus at the clypeal fossa.

Copulatory Male. A fully adult male identified by having a sperm tube of the second pleopod's stylet that is open at its sharp distal tip. In some specimens at this terminal stage, the vas deferens connecting to the penile papilla is visible through the cuticle.

Coxa. The first or basal segment of a thoracic appendage. See pereopod.

Cuspate. Having a sclerotized surface or margin with one or more rounded projections.

Cuticular Combs. Tiny arc shaped or linear groups of cuticular spines, most easily seen on the distal parts of the mandibular palp, but may occur elsewhere on the cephalic appendages.

Cuticular Organ. The paired female copulatory organ of Asellota, found either ventrally or on the anterior dorsal margin of pereonite 5.

Cuticular. Of the cuticle.

Dactylus. The seventh or distal segment of a thoracic appendage, bearing one or more distal claws. See pereopod.

Denticle. A short, pointed, tooth-like projection of the cuticle.

Denticulate Seta. A generally robust seta with either a row of denticles or a group of distal denticles.

Denticulate. Having denticles.

Dorsal Orifice, male pleopod I. The distal opening of the sperm tube in the janiroidean male first pleopod.

Dorsum (plural Dorsa). The dorsal surface of a body segment.

Endopod. The medial or interior ramus of a crustacean appendage. In the Isopoda, another name for a thoracic appendage (exclusive of the coxa and basis), although more typically applied to inner ramus of a pleopod or a uropod.

Epimere. A lateral fold of a somite's integument dorsal to the limbs. Sometimes called the pleurite or tergal fold.

Epipod. Laterally directed lobe (exite) of the basal segment (coxa) of the maxilliped.

Exopod. The lateral or exterior ramus of a crustacean basis. In the Isopoda, applied to the outer ramus of a pleopod or a uropod.

Facies. An appearance or similarity, as in Ilyarachnoid Facies.

Fan Seta. A specialised seta on the distal tip of the maxilliped's endite. It is made of thin, hyaline cuticle (difficult to see) and is usually broad with many laterally pointed lobes. It may appear as two distinct types: a medial, more heavily sclerotized seta with fewer lobes, generally found on the distomedial corner of the maxillipedal endite; and a thin lamellar form placed in a row just proximal to the distal edge of the endite.

Flagellum (plural flagella). The long, tapering distal part of either the antennula or antenna, generally made of many annuli.

Foliaceous. Leaf-like.

Foregut (synonym Stomodeum). The crop-like anterior portion of the gut that is lined with cuticle and has openings to the lateral digestive caeca and the posterior midgut.

Fossosome. A hardening and fusion of the anterior pereonites 1-3 with a spade-like head inserting flexibly into the first pereonite. The first 3 pereopods are robust and curve laterally with posterior-angled setae. This apomorphy of the Macrostylidae is presumed to be an adaptation for burrowing (see Thistle and Wilson, 1986, 1997).

Fossa. A ventral trough in the clypeus into which the mandible's condyle articulates.

Frons. The anterior part of the cephalon bearing the clypeus and lying between the antennulae and antennae and below the rostrum or vertex.

Frontal Arch. A thickening of the cephalic frons that provides a strengthened arch between the fossal regions of the clypeus on either side of the frons. Generally associated with enlarged and heavily sclerotized mandibles. See Wilson (1989, fig. 38).

Geniculate. Knee-like, or displaying an acute angle between two segments. As in geniculate segments 2 and 3 of the antennula.

Gnathopod. First or second pereopod differing in function and appearance to following pereopods, usually chelate or subchelate and used for manipulation of food.

Gravid. Bearing fully formed ova or embryos in the ovary. This is the condition of fully mature preparatory females.

Habitus. Appearance of the whole animal.

Hemiplumose. A modified form of the plumose seta in which closely spaced long setules are found in only one row.

Hindgut (synonym Proctodeum). The posterior portion of the gut connected to the anus and lined with cuticle.

Incisor Process  (synonym pars incisivus). The distal biting part of the mandible that typically bears one or more pointed cusps. On its medial side, it bears the spine row.

Indurate. Heavily sclerotized or calcified, and often rough.

Instar. A discrete stage in a growth series, delimited by successive moulting.

Interantennular. Between the antennae.

Ischium (plural Ischia). The third segment of a thoracic appendage. See pereopod.

Labrum. An unpaired, flat segment of the cephalon that articulates with the clypeus, and anteriorly covers the mandibles.

Lacinia Mobilis (or lacinia). An enlarged, nearly articulated spine of the mandible's spine row that is adjacent to and plesiomorphically resembles the incisor process. It is found on the left mandible. On the right mandible, it is replaced by a large spine similar in shape to the more posterior members of the spine row. (see Richter et al., 2002)

Lamella. A broad flattened appendage.

Locking Folds, Dorsal. Paired projections of the dorsal cuticle of the male first pleopods in the Janiroidea; these structures are absent in other Asellota. They form a seat for the medial edge of the second pleopod protopods, allowing both pairs of pleopods to function together as an operculum.

Lower Lips. See Paragnaths.

Manca. One of the first three stages or instars of an isopod's postmarsupial life cycle, wherein the seventh pereopod is absent or rudimentary. In certain neotenic Asellota, this condition is retained in the adult, in which the manca stage cannot be identified by these criteria.

Mandible. The third cephalic appendage, and first mouthpart appendage of isopods. It generally has a lateral three-articled palp and is made up of the following functional regions: incisor process, spine row, molar process, dorsal condyle, and posterior articulation.

Marsupium. A ventral pereonal enclosure on females for developing embryos. It is composed of oostegites projecting medially from the coxae of the anterior pereopods (pers 1-4 in asellotan isopods). Some isopods have an additional small oostegite on the coxa of the maxilliped.

Maxilla (plural maxillae, synonym second maxilla). The third paired mouth part and fifth cephalic appendage. In the Janiroidea (Asellota: Isopoda), it consists of a basal segment bearing three setose lobes.

Maxilliped. Paired appendage on the posterior and ventral edge of the cephalon. Actually it is the first thoracic appendage, but its body somite is fused into the cephalon, and it is modified for feeding. It consists of the following functional parts: coxa, basis bearing a flattened and setose endite, palp with 5 segments (ischium, merus, carpus, propodus, dactylus), and epipod attached laterally to the coxa.

Maxillula (plural maxillulae, synonyms maxillule, second maxilla). The second mouth part and fifth cephalic appendage. In the Janiroidea, it consists of two setose lobes: a large outer lobe armed with robust, tooth-like setae; and a smaller inner lobe with only small setae.

Merus (plural Meri). The fourth segment of a thoracic appendage. See pereopod.

Midgut. The central region of the crustacean gut. Unlike the fore and hind gut, this region lacks cuticle.

Molar Process (synonym pars molaris). A medial process of the mandible. Primitively it has a broad, distal, triturating surface with circumgnathal denticles, a posterior row of broad, setulate setae, and sensory pores on the distal surface.

Natapod. A natatory pereopod of the Munnnopsidae, pereopods V - VII.

Natasome. The often posteriorly streamlined body section of the Munnopsidae consisting of the following body segments: heavily muscularised pereonites 5-7, and the pleotelson.  See also “ambulosome” for the anterior somites.

Natasomite. A pereonite of the natasome in the Munnopsidae.

Oopore. A paired female opening in the ventral cuticle of pereonite 5, through which the fertilised ova are released via the oviduct into the marsupium.

Oostegites. Lamellar lobes of cuticle extending medially from the coxa of an adult female isopod. They may be seen in two forms: developing oostegites are small fat lobes that do not cross the ventral midline; oostegites of the brooding female are broad, long lamellae that overlap on the ventral midline, forming a marsupium for the developing embryos.

Operculum/Opercular. A covering / forming a covering. In female Janiroidean asellotan isopods, the operculum is a plate over the branchial chamber of the abdomen consisting of the fused second pleopods (the first pleopods are absent in these isopods).

Oviduct. An often complex female organ connecting the ovaries to the oopores. In the Asellota, it consists of the following functional subsections: outer tissues surrounding internal parts; spermatheca, which may or may not be covered with cuticle; and cuticular organ, an often complex cuticular tube.

Ovigerous. Bearing developing embryos in the marsupium. (See also gravid).

Palp. A lateral appendage of the mandible or the maxilliped.

Paragnaths (synonyms Paragnathae or Lower Lips). A pair of ventral projections of the cephalic cuticle just posterior and medial to the mandibles. It consists of two pairs of lobes, a broad lamellar outer pair with hair-like setae on their inner margins and a thick inner pair covered with many hair-like setae.

Paucisetose. Having few setae.

Pedestal Seta. A robust seta that is raised above the dorsal surface of the body by a pedestal-like outpocketing of the cuticle.

Penicillate seta (synonym “broom seta”). A sensory seta that has a distinct articulated pedestal, and two distal rows of long, extremely thin setules. It may be found on the antennulae or any of the pereopods.

Penile Papillae (synonym Penes). Male cuticular projections on the posterior and medial margin of the seventh pereonite. They contain the openings of the vasa deferentia.

Pereon. Thoracic segments 2-8 bearing the locomotory appendages, or pereopods. (Thoracic segment 1 is part of the cephalon and bears the maxilliped).

Pereonite. A segment of the pereon.

Pereopod. One of the seven pereonal appendages. Consists of the following segments: coxa, basis, ischium, merus, carpus, propodus, dactylus. The coxa of adult female bears oostegites. The distal five podomeres are homologous with the endopod of the more primitive biramous thoracic limb of other Crustacea.

Pleonite. A segment of the abdomen. Primitively in the Malacostraca there are six free pleonites and a telson, in isopods the sixth pleonite is fused to the telson to form a 'pleotelson'.

Pleopod. One of the first five paired, biramous, ventral limbs of the pleotelson. In unmodified form, it consists of a basal segment, the protopod, and two distal rami, the endopod and the exopod. The rami may be biarticulate. Female Asellota lack the first pleopods. In male Asellota, the first pleopods are present only as uniramous structures (fused into a single elongate plate in the superfamily Janiroidea). The rami of the male second pleopod are modified as copulatory structures. Pleopods III-V have very thin cuticle and function as gills (branchiae).

Pleopodal Cavity. The deeply concave ventral surface of the pleotelson that encloses the pleopods dorsally and laterally. Because the more posterior pleopods function as gills it is sometimes called the branchial cavity.

Pleotelson. Segment resulting from fusion of the sixth pleonite with the telson.

Plumose seta. A feather-like seta that has two dense rows of thin, long setules beginning at the base of the seta and continuing to the tip.

Podomere. A segment of a crustacean appendage.

Preanal Ridge. A raised transverse ridge on the ventral surface of the pleotelson situated between the pleopodal (or branchial) cavity and the anus. In some Munnopsidae, this ridge becomes large and projects ventrally.  In other families (e.g. Desmosomatidae, Haploniscidae, Ischnomesidae), the ridge is a broad bar separating the pleopodal chamber from the anus.

Prehensile. Adapted for grasping or clinging, movable finger consisting of the dactylus, or dactylus & propodus and palm consisting of either the propodus or the carpus (the latter being the typical state of the Janiroidea).

Preparatory Female. An adult female that has developing oostegites.

Propodus. The sixth segment or podomere of a thoracic appendage. See pereopod.

Protopod (synonym protopodite). The basal segment of the pleopods and the uropods. It consists of the fused coxa and basis of the crustacean limb. In a general crustacean nomenclature, the protopod includes all basal podomeres up to and including the segment with the rami. The antenna, therefore, consists three protopodal segments.

Quadrangular (synonym quadrate). Having a truncate distal margin at approximately right angles to the lateral sides.

Ramus (plural Rami). A branch of an appendage.

Receptaculi (synonym Coupling Hooks). Modified setae on the medial margin of the maxilliped's basal endite that have bulbous recurved and denticulate tips. They couple with their paired counterparts so that both maxillipeds can act as a single unit.

Recurved. Curved back on itself.

Rostrum. A projection of the cephalic frons that may also include the dorsal surface of the cephalon.

Sclerotized. With thick and sometimes calcified cuticle.

Sensilla. A modified seta found on the dactylus of the pereopods. It is similar to an aesthetasc, but has a heavier cuticle that is covered with many tiny lobes (often only visible in a scanning electron micrograph).

Sensory Pore, Mandibular Molar. A small pit in the distal surface of the mandible's molar process that can be seen to connect internally to a nerve process.

Serrate. Having a row of short tooth-like denticles or spines.

Seta (plural Setae). A cuticular process that is clearly articulated with the basal cuticle. This structure comes in many forms. Some papers in the isopod literature may use the term “spines” when heavily sclerotized setae are indicated. "Robust seta" or "spine-like seta" for such heavy setae is more accurate.

Setulate Seta. A seta with one or more rows of setules. It is different from plumose or hemiplumose setae in that the row is limited to a section of the shaft, and does not extend from base to tip.

Setule. A thin spine on a seta.

Sperm Tube. A structure found only in the pleopods of male janiroidean Asellota, refering to two forms: 1. A cuticular tube in the stylet (distal segment of the endopod) of the male second pleopod, consisting of a ventral opening to a rounded chamber in the centre of the stylet and a confluent tube to the tip of the stylet. 2. A medial cuticular tube between the first pleopods that opens anteriorly at a funnel-like chamber for the penes, and posteriorly and dorsally above the opening of the stylet sperm tube. During copulation, both tubes may form a single channel from the penile papillae to the female's cuticular organ. (see Wilson, 1991).

Spermatheca. A sperm reservoir inside the female oviduct with an opening to the cuticular organ.

Spine Row, Mandibular. A row of spines on the medial side of the mandible's incisor process. The lacinia mobilis on the right mandible may be an enlarged member of the spine row (see Richter et al., 2002).

Spine. A pointed outpocketing of the cuticle that is confluent with the cuticle at its base (not articulated), see also seta.

Statocyst. Small saclike sensory organ, usually containing a granule(s), used to indicate to the animal its orientation.

Sternite. The ventral surface of a thoracic body segment.

Subchelate. Having the functional ability to grasp by folding together two adjacent podomeres of a limb.

Support Ridge, Posterior, Mandibular. A cuticular ridge on the body of the mandible that is a continuation of the dorsal condyle, but does not articulate with the fossa in the clypeus.

Supraclypeal. Above the clypeus.

Sympod (synonym Protopod). A appendage segment made of the fused basis and coxa.

Telson. The terminal segment of a crustacean's body, bearing the anus. In isopods, the telson is fused to the anterior pleonite(s).

Tergite. The dorsal surface of a body segment.

Thoracic. Of post-cephalic segments 1 through 8.

Tridentate. With three denticles.

Triturating Surface. The truncate distal surface of the mandible's molar process that opposes the same surface on its counterpart.

Triturative (synonym “gnathal” ). Of the biting or grinding surface on the molar process of the mandible.

Unequally Bifid Seta. A seta that is often spine-like and has a smaller sensilla or hair just proximal to its tip. The hair can be seen to have a nerve extending into the cuticle and is probably the external expression of a sensory nerve. Most true setae are of this form, but may not have an easily seen sensilla.

Unguis (synonym Claw). A modified seta on the tip of the dactylus.

Uniarticulate. With only a single segment.

Uniramous. With only a single branch.

Uropod. The terminal appendage of the body, belonging to the sixth pleonite. It consists of a basal segment, the protopod, and primitively two uniarticulate rami, a larger endopod and a smaller exopod.

Vas Deferens. Male duct from the testis to the penes for the passage of sperm.

Venter (synonym sternum). The ventral side of the body.

Vertex. The anterior and medial margin of the cephalic dorsal surface.

Whip seta. Similar to the unequally bifid seta, except that it is generally more slender, and the sensory hair is on the distal tip and is long and curved.