TYPE CRUSTACEA. 



415 



forwards only a short distance into the thorax segment. It 

 possesses one or two pairs of ostia and is closed behind, 

 giving off in front and at the sides numerous aortse. A shell- 

 gland has been observed in some Isopoda, but the auteunary 

 gland is wanting. 



Although the majority of the marine forms, such as Idotea 

 and Spha&roma, lead a free existence, nevertheless there are 

 certain parasitic forms. Thus the genera Cymothoa and JEga 

 are parasitic on the skin or in the mouth of fishes, but also 

 retain the power of swimming and consequently are not much 



FIG. 189. Asellus communis, DIAGRAM op STRUCTURE. 



ab = abdominal appendages. I = liver-coeca. 



ao = aorta. mnp = inaudibular palp. 



at 1 = antennule. mxp = maxilliped. 



at 1 = antenna. r = rectum. 



ce = cerebral ganglion. s = stomach. 



ch = chelate limb. t = thoracic appendage. 



fit = heart. vn = ventral nerve-cord. 



modified. The genus Bopyrus, which lives in the branchial 

 cavity of shrimps, becomes in the female somewhat distorted 

 in shape and asymmetrical, and the mouth-parts become 

 transformed into a suctorial proboscis and the eyes disap- 

 pear. The male, however, which is much smaller than the 

 female, retains the eyes and does not depart from the usual 

 symmetrical body form. The degeneration of the female 

 proceeds much farther in the genus Entoniscus, which lives 

 either partly or wholly included within the body-cavity of 

 other Crustacea and assumes a saclike unsymmetrical form, 

 recalling to a certain extent that of some of the parasitic 

 Copepoda. At the time of pairing both sexes are alike fully 

 segmented and with an almost full complement of appendages. 



