412 ARTIIROPODA. 



together only in the larvae; in the adult one or the other is sup- 

 pressed. In some amphipods there are excretory diverticula 

 developed from the intestine (fig. 448), which resemble the Mal- 

 pighian tubes of insects, but differ from them in being of ento- 

 dermal origin. In some decapods caeca occur in the same region, 

 but nothing is known of their function. 



Visual organs are either represented by the so-called nauplius 

 eye, consisting of a pigment spot with three lenses situated directly 

 on the brain, or by a pair of compound eyes. The nauplius eyes 

 are chiefly found in the lower, the compound in the higher, groups; 

 occasionally they coexist in the same species. Auditory (equili- 

 bration) organs (otocysts) occur only in the Malacostraca, either 

 in the base of the first antennae or in the endopodite of the last 

 abdominal feet (fig. 434, o). These are rarely vesicular, but are 



FIG. 414. Otocyst of crayfish, as, auditory ridge; ?i, nerve. 



usually grooves (fig. 414), bearing at the base a row of chitinous 

 sense hairs, the crista acustica, connected below with an auditory 

 nerve, while their free ends extend between a cluster of otoliths. 



At ecdysis these otocysts with their sensory hairs and otoliths are 

 cast off. If a crayfish which has just molted be placed in perfectly clean 

 water, the otocyst will remain without otoliths; but if some easily recog- 

 nizable substance, like uric acid crystals, be placed in the water, some of 

 these will soon be found in the sac, thus proving that the otoliths are 

 introduced from the outside. 



Crustacea are only exceptionally hermaphroditic. The sperma- 

 tozoa are noticeable for their great size, in many ostracodes equal- 

 ling the body in length. Except in the Cirripedia the spermatozoa 

 lack a flagellum and are immobile. Their round or elongate body 

 is covered with rigid processes reminding one of the pseudopodia 

 of Actinosphcerium (fig. 36, y, 6). They are frequently enclosed 

 in spermatophores (fig. 422). 



The typical development of a crustacean includes a metamor- 

 phosis, and where direct development occurs the metamorphosis is 



