Nucleated yellow brown pigment cells intervene between the retinulae, and two (or more) black pigment cells between the crystalline cones. The ectoderm cells at the margin of the eye are elongated and filled with black pigmentl.
The protopodite of the first antenna is three-jointed. The basal joint is tri-hedral and lodges the auditory sac. The exo- and endo-podite are both annulated. The number of joints in the exopodite vary, but, distal to the first eight, they bear on their ventral surfaces the olfactory setae so-called. The auditory sac is curved, and possesses delicate chitinous walls. Its aperture, which is permanently open, but is protected by numerous setae springing from its outer margin, lies in the dorsal surface of the joint. The auditory setae are arranged in two rows which meet at the closed end of the sac. The largest setae are one-fiftieth of an inch long. They are hollow and are moveably attached to the wall of the sac. This attachment is by a membrane delicate on one side, stout on the other forming the 'tooth' of Hensen. A narrow plate, the 'ligula' of Hensen, is developed in the shaft of the setae, on the side opposite the tooth. The shaft with the exception of the ligula is beset with fine solid barbs. The nerve fibre is furnished with a ganglion cell close to its termination, which is fine and delicate, and according to Hensen attached to the ligula. The otoliths are numerous and irregular in shape.
According to Hensen's observations on Paiaemon antennarius, these otoliths are foreign particles collected by the forceps and scattered over the base of the first antenna, whence some find their way into the sac. A specimen which moulted and was confined in a basin of filtered sea-water was supplied with crystals of uric acid. Whilst the cast-off auditory sac contained the usual otoliths, the new sac contained a large proportion of uric acid crystals. The foreign bodies thus obtained are kept in situ apparently by a gelatinous substance. When the auditory sac is a closed one, there are either no otoliths (Brachyura), or e. g. in Mysis, there is one otolith in the shape of a rounded laminate body, apparently a secretion.
1 For the terms descriptive of the structures found in the eyes of Arthropoda see the general description of that phylum. The Crayfish's eye would be described, using the terms there given, as polymeniscous, diplostichous, retinulate, furnished with vitrellae, and perhaps also as exo-chromic. But the origin of the pigment cells has not been exactly ascertained. See Lankester and Bourne, Q. J. M. xxiii. 1883, on Eyes of Scorpio and Limulus.
The olfactory hairs occur 2-3 on the first two joints that possess them, the subsequent joints having an anterior and posterior row with 7-8 hairs in a row. They are about 1/200 of an inch long, 'shaped like a spatula with a rounded handle and somewhat flattened blade' (Huxley). They are two-jointed, and contain a soft granular tissue. Jourdain, who has recently investigated these structures in various Crustaceans, states that a small hyaline body projects from the free extremity and that a nerve fibre is traceable to the base of each hair, and may sometimes be seen to have a swelling (? ganglion cell). He terms the hairs 'poils a batonnet,' and divides them into 'poils a batonnet cylindriques et a batonnet stipites.' The former are long and cylindrical and usually many-jointed, the latter are usually three-jointed and somewhat fusiform. According to him the Crayfish possesses the cylindrical variety, but the hairs are short.
The protopodite of the second antenna is two-jointed. The basal joint bears a ventral tubercle, to the inner side of which is the aperture of the green gland. The second joint is divisible into two parts more or less moveable, and bears an exopo-dite in the shape of a scale or 'squame.' The endopodite is long and many-jointed.
The mandible or first appendage of the mouth consists of four joints. The basal (= coxopodite) is long and forms a three-sided pyramid. The base of the pyramid projects inwards over the sides of the mouth. The oral side of the base bears two stout obscurely separate teeth; the posterior side and outer angle form a sharp ridge with several teeth, but the anterior side is hollowed out, the hollow invading the centre of the base. The other three joints are small, the terminal dilated and fringed with setae. They are articulated to the anterior side of the pyramid not far from the base. They constitute the 'palp,' which is not an exopo-dite, a structure rarely present in the mandible of the adult, as e.g. in some Cope-poda. In the larval form known as Zoaea, the first Zoaea-stage has no palp to the mandible. It sprouts out in later stages. In the Nauplius the mandible has the typical biramose character, but when Penaeus, the only Decapod with a Nauplius stage changes to the Zoaea, the mandible is reduced to its basal portion, and the palp is evolved at a later period. The Phyllopod mandible is similarly reduced, and never gains a palp.
But the temporary suppression of a limb or part of a limb is by no means an uncommon phenomenon in the higher Crustacea (cf. the account in Balfour's Comparative Embryology, i., of the evolution of Sergestes, p. 398, Phyllosoma, p. 396, and Squilla, pp. 402-3).
The other appendages of the mouth are best taken in reverse order. The most perfect is the third maxilliped. The protopodite is divisible into a coxopodite which bears a podobranchia and coxopoditic setae (cf. p. 182) and a basipodite. This joint, as in the forceps, is continuous with the basal joint of the endopodite which is divisible into an ischiopodite, meropodite, carpopodite, propodite and terminal dactylopodite. The exopodite or palp is short and articulates with the basipodite. It is slender, and has a long basal joint and a many-jointed filament. The second maxilliped has a smaller and softer endopodite, and a larger exopodite. There is a podobranch but no coxopoditic setae. The basipodite is distinct, and the meropodite very long. The first maxilliped has the coxopodite and basipodite imperfectly separate and expanded into thin setose plates. The endopodite is short and two-jointed; the exopodite large, with a much elongated basal joint. The podobranch is reduced to the stem and lamina, and is known as the flagellum or epipodite of Milne-Edwards. The three pairs of maxillipeds belong to the thorax.