300 A HISTORY OF EECENT CRUSTACEA 



lateral angles of the carapace close it in only at tbe sides 

 and not in front. 



The marHupium of the female is formed by the splitting 

 of the ventral wall of the segments which carry the third 

 maxillipeds and the first three pairs of peraeopods, the 

 upper lamina separating the pouch from the general body- 

 cavity. Only in the aberrant Isopod genus Gnathia 

 (wrongly called Anceiis) has any correspondent arrange- 

 ment for the marsupium been observed, although there the 

 ventral wall serves the purpose without being split into 

 two laminse. 



The body contains an oval or spindle-shaped heart, ex- 

 tending from about the middle of the carapace back 

 through two or three of the free segments. It has one 

 pair of lateral venous openings. At the sides of it are two 

 elongate organs with some excretory function. The liver- 

 tubes are usually three pairs, but there is only a single 

 pair in the family Campylaspidse. The nervous system 

 comprises the supraoesophageal ganglion or brain, and 

 sixteen ganglia, of which the first three in close contiguity 

 innervate the parts of the mouth. The next seven belong 

 to the trunk and are connected by rather long double 

 commissures. The remaining six, pertaining to the pleon, 

 are weakly developed, and their connecting commissures 

 are very long and slender. 



It is a rather singular circumstance that the telson is 

 large and conspicuous only in two out of the eight families. 

 In one other family it is distinct, but small and without 

 spines. In the remaining five families it is entirely want- 

 ing, except so far as the ventral opening may vouch for its 

 existence, and a slight prolongation of the sixth pleon- 

 segment may suggest that it is not wholly lost but 

 coalesced. 



Of the characters given the combination of a very few 

 will suflBce to distinguish the Cumacea from all other 

 Edriophthalma. 



