74 



THE CRUSTACEA 



four somites with the telson constitute the abdomen, and are limb- 

 less, with the possible exception of the first or genital somite, which 

 bears, in both sexes, the external openings of the genital organs, 

 and may have what are sometimes regarded as the vestiges of a 

 pair of appendages. 



In the majority of cases the number of free somites is reduced 

 by coalescence. Very often the second (commonly reckoned as 

 the first) thoracic somite, bearing the first pair of swimming-feet, is 

 fused with the head, and not infrequently (in the Gymnoplea) the 



Fio. 41. 



A, Calanusfinmarchicus, female, from the side, x 16; B, antenna : C. maxilla; D, maxilliped. 

 ii and vi, second and sixth thoracic somites ; 1+2, first and second alxlominal somites coalesced ; 

 a', antennule ; en, endopodite ; ex, exopodite ; /, caudal t'urca. (After Sins.) 



last two thoracic somites fuse together. The abdominal somites 

 are usually all distinct in the male sex (Fig. 40, B), but in the 

 female this is rarely the case, the first two being generally fused 

 (Figs. 40 and 41, A), and the number may be still further reduced. 

 While there is no distinct shell-fold, the head-region usually 

 possesses more or less well-developed pleural folds, which may be 

 directed downwards or even inwards so as partially to enclose the 

 bases of the limbs. These pleural folds are repeated on the free 

 thoracic somites, but are as a rule absent on the abdomen. An- 

 teriorly the tergal plate of the head-region is often produced into 

 a rostral process which may be a flattened plate, movably articu- 



