The second maxilla has a very thin lamellate coxopodite and basipodite, each partially subdivided by a fissure. The endopodite is small and simple. The exopodite forms a large plate, the scaphognathite, which is kept in perpetual motion and bales the water out of the branchial chamber, into which it runs posteriorly and inferiorly. For its homology see Claus, Untersuchungen zur Erforschung der genealogischen Grundlage des Crustaceen-systems, Wien, 1876, p. 42. The first maxilla has two thin simple and foliaceous expansions. The first represents the coxopodite; the second is usually termed basipodite, but the homology is doubtful. The endopodite is extremely reduced, and the exopodite absent. It is present in Euphausia and in the Zoaea of Penaeus up to a certain stage, and then atrophies.
The sides of the mouth are formed by a soft lip. In front it is overhung by a leaf-like projection, the labrum, attached basally to the epistoma or broad triangular calcified area in front of the mouth and extending forwards to the base of the antennae. The sides of the mouth are overhung by the bases of the mandibles, and behind the latter are two small soft lobes united by the posterior margin of the mouth. These lobes are the paragnatha, metastoma, or lower lip. Neither labrum nor metastoma represent appendages in development, but it is possible that the latter represents a dissociated portion of the first maxilla (Claus, p. 15, Neue Beitrage, etc, infra). The ventral region between the bases of the mouth parts, and again between the bases of the first and second antennae becomes calcified, forming a series of sterna. The antennary sternum constitutes the epistoma.
The rostrum in Nebalia is a moveable process. In Squilla it is jointed to the fore-edge of the carapace and moves with the somite of the first antenna. In Nebalia it appears to belong rather to the region of the first than to that of the second antenna, to which Professor Huxley allocates it.
The ventral and basal lateral parts of the sternal wall of the mouth-parts and thorax give origin to a number of internal cuticular folds or apodemata. These apodemata constitute the endophragmal skeleton and give attachment to muscles as well as protection to the thoracic portion of the nerve-chain and the sternal blood-sinus. Their arrangement is too complicated to be explained without the aid of figures, and the student may consult Professor Huxley on the Crayfish, p. 157, where will be found both description and figures.
There remain for examination the two first pairs of abdominal appendages in the male and the telson.
The first appendage is unjointed and cylindrical. Its apex forms a plate slightly bifid. The sides of the plate are rolled upon themselves, the anterior half surrounding the posterior, giving rise to a canal open at each end. It is not certain what parts of the typical limb are represented in this appendage.
The second appendage has a protopodite divisible into a coxopodite and long basipodite. The endopodite consists of a large basal piece and terminal jointed filament. The apex of the basal piece is prolonged upwards as a plate to the inner side of the filament, and the inner edge of the plate is rolled upon itself. The exopodite is present and has the usual structure. Both pairs of appendages are used for the transmission of sperm: see p. 186.
Both the male and female alike possess no appendages to the first abdominal segment in the Parastacidae - the Crayfishes of the S. Hemisphere.
The telson is a plate moveably articulated to the last somite of the abdomen. It is divided in nearly all the Potamobiidae or Crayfishes of the N. Hemisphere, but not in the Parastacidae, by a transverse suture, so that the posterior half is moveable upon the anterior half. The anus is situated on the ventral surface of the basal portion. There is some question as to what the telson represents. Hartog (British Assoc. Reports, 1882, p. 575) believes that it represents the last somite of the Nauplius - a post-anal plate united to the furcae anales, the latter representing paired terminal outgrowths elsewhere developed into limbs by the formation of joints. He points out that in the Copepoda - in his view the primitive Crustacean group - the anus is a terminal dorsal slit; that the tergum of the last somite forms a supra-anal plate, whilst the furcae project one on either side of the anus. Supposing the supra-anal plate to become adnate to the furcae, the anus becomes first terminal and ventral, and finally by growth of the plate ventral. He homolo-gises two setose knobs projecting at the sides of the telson in Astacus with the furcae.
It has been pointed out by Claus (Untersuchungen, etc. supra, p. 12) that in the Protozoaea-stage of Penaeus the anus is terminal between two furcal processes. In its youngest Zoaea a short transverse bridge (=supra-anal plate of Copepoda)) connects the furcae dorsally above the anus. During subsequent growth this bridge enlarges, the anus becomes more and more ventral, and the furcae are lost, becoming the two posterior setigerous processes of a broad bilobed terminal plate. In a young Phyllosoma (cf. Claus, op. cit. p. 51) the same facts may be observed, but the processes become obsolete, the setae alone persisting at the outer angles. There is much variety in the shape of the last somite of the abdomen in Crustacean larval forms. It is very frequently a broad bilobed plate, more rarely, as in the young Astacus itself, a simple plate. In both cases the anus is ventral. Claus concludes that the telson represents the terminal furcal somite of the abdomen in Phyllopoda. It is possible that it may represent a region rather than a somite.
For in Nebalia (Claus, Z. W. Z. xxii. 1872, p. 329) there are two somites behind the sixth abdominal somite, the last bearing a pair of furcae and the anus, and in most Phyllopoda the abdomen contains a large number of somites.