COMMON CRAYFISH. 169 



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. 



The olfactory hairs occur 2-3 on the first two joints that possess them, the sub- 

 sequent joints having an anterior and posterior row with 7-8 hairs in a row. They 

 are about -3^ 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 stipiteV 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 in- 

 vading 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 



