TYPE CBUSTACEA. 383 



surface of tlie abdomen, and appear to be rather phospho- 

 rescent than optic organs. 



Otocysts occur throughout the group Decapoda, to which 

 the Crayfish, Lobster, and Crab belong, and consist of sacs 

 lined by sensory setae and containing otoliths. They are 

 situated on the basal joint of each of the antennules and in 

 some forms are completely closed, though usually their cavity 

 communicates with the exterior, being guarded by a number 

 of closely-approximated bristles. In the Schizopoda similar 

 otocysts occur in the endopodite of the last pair of abdominal 

 appendages, and in the Amphipod Oxycephalus two lie above 

 the syncerebrum. These structures, which are usually spoken 

 of as auditory organs, seem to be rather sense-organs of equi- 

 librium. 



In the larvse of many forms and in the adults of some 

 Entomostraca one or two papilla-like processes project from 

 the anterior surface of the head and are supposed to be sen- 

 sory in function, though what purpose they may subserve is 

 unknown. Strong nerves pass to these frontal sense-organs 

 which appear to be of considerable importance. 



The excretory system consists of two pairs of nephridia, 

 one or other of which may be absent in many forms. One of 

 these develops in connection with the antennary segment and 

 opens to the exterior on the basal joint of the antennae (Kg. 

 168, he), whence it is known as the antennary gland, some- 

 times, however, receiving the name of the green gland. It 

 reaches its highest development among the Malacostraca, 

 occurring in many Entomostraca only in larval stages, later 

 on degenerating. In its simplest condition it consists of a 

 coiled tube whose lumen appears in some cases to be intra- 

 cellular, though in others it is undoubtedly intercellular, and 

 which terminates internally in a saclike dilatation whose 

 wall is richly supplied with blood-lacunae. In the higher 

 forms (Fig. 172, A) a great complexity is brought about by 

 the development of lateral branches from the tubular portion, 

 and the terminal sac (s) may enlarge and fuse with tbat of 

 the opposite side to form a cavity of considerable size lying 

 in the anterior portion of the thorax and termed the nephro- 

 peritoneal sac. 



