430 



CRUSTACEA. 



(fig. 341), two toothed, usually palped mandibles are placed be- 

 neath the upper lip. These function in the free-living Copepoda as 

 masticatory organs, but in the parasitic forms are usually trans- 

 formed into pointed styliform rods, which are used for piercing. 

 In this case they are frequently placed in a suctorial tube formed by 

 the junction of the upper and under lips. The two jaws which 

 follow the mandibles are weaker biting plates, and in the parasitic 

 Copvpoda are reduced to small palp-like protuberances. The maxil- 



lipeds, on the contrary, are much longer ; 

 they are used to procure food and, especially 

 in the parasitic forms, to attach the body. 

 The thoracic swimming feet consist of a 

 two-jointed basal portion, and two three- 

 jointed setigerous swimming raini, which 

 are comparable to broad swimming plates. 

 In the ArgulidfK these rami are much 

 elongated, and by their numerous joints 

 approximate to the legs of the Cirripedia. 

 Nervous System. In all cases there is 

 a brain giving off sensory nerves, and also 

 a ventral cord, which either develops 

 some ganglia in its course or is concen- 

 trated to a common subcesophageal gan- 

 glionic mass. Of sense organs the median 

 frontal eye, divided into three parts (Cy- 

 clops eye), is pretty generally present. 

 The tactile sense is specially localized in 

 the sete of the anterior antenna?, but is 

 probably also present in many other parts 

 of the body. Olfactory hairs are pre- 

 sent as delicate appendages of the an- 

 terior antenna?, principally in the male 

 sex. 



The alimentary canal is divided into 

 a short narrow oesophagus, a wide sto- 

 mach which often has two blind diverticula near its commence- 

 ment, and a narrow rectum which opens on the dorsal surface of the 

 last abdominal segment. The surface of the intestine often seems 

 to perform the function of a urinary organ. We find, however, 

 at the same time a shell gland in the cephalo-thorax at the sides of 

 the maxillipeds. In all cases the whole surface of the body performs 



FIG. 341. Mouth parts of 

 Cyclops. M, Mandibles ; MX, 

 maxilla; Kf, internal; Kf" , 

 external niaxilliped. 



