17: 



THE CRUSTACEA 



mandibles. The possible homology of this groove with the 

 cervical groove of Anaspides has been already mentioned. The 

 abdominal somites have the pleural plates generally reduced or 

 absent ; but in Gastrosaccus the pleura of the first somite are greatly 

 enlarged in the female to help in forming the brood-pouch. The 



Fig. 90. 

 Mijsis relicta, female, c.s, cervical sulcus ; m, brood-pouch. (After Sars.) 



sixth somite is generally longer than any of the others, and in 

 Gmdhophaiisia it is divided by a transverse groove (Fig. 100, gr) 

 about the middle of its length. It is possible that we have here 

 two somites in process of coalescence, and that seven somites are 

 represented in the abdomen of the Mysidacea as in that of the 



Gnathophausia willemoesii, Sars. (According to Ortmann this is the fully adult form of (!. 

 zoea, W. Suhm.) x ^. gr, groove partially dividing the last somite. (After Sars.) 



Leptostraca. It is important, however, to note that the last somite 

 in this case bears appendages (uropods) and that the penultimate 

 does not, while in the Leptostraca the reverse is the case. The 

 indications of a similar division of the sixth abdominal somite in 

 certain fossil Syncarida have already been alluded to. 



Appendages. — The ocidm- peduncles are peculiarly modified in 

 many deep-sea forms in which the eyes are imperfect or absent. 

 In Dadylenjtlirops the peduncle is produced as a finger-like process 



