394 



CRUSTACEA ENTOMOSTRACA. 



the cuticular skeleton, or both, 

 is fairly constant, though depar- 

 tures from the usual number are 

 frequently met with, especially 

 in the abdomen, and the number 

 may differ in the sexes of a 

 single species. 



The cephalothorax terminates 

 in front in a ventrally directed 

 pointed or forked rostrum. It 

 bears two pairs of antennae, 

 mandibles, two pairs of maxillae 

 and a pair of maxillipeds. Pos- 

 teriorly it may be distinct (some 

 Calani d a e ) 

 from the 

 second tho- 

 racic seg- 

 ment (Fig. 

 260), but 



Flo. 258. An anterior antenna of the generally 

 male Cyclops serrulatus (after Claus). j_ 

 Sf olfactory organs, M muscles. t W O 



terior tho- 

 racic segments are fused with the head. 

 Behind the second come four other tho- 

 racic segments, bearing swimming feet, 

 but the two last may be fused together. 

 In the females of many Calanidae the 

 sixth thoracic feet are absent, and in the 

 Cyclopidae and Harpactidae they are 

 rudimentary, the segment to which they 

 belong being included, as we have seen, 

 in the urosome. 



From the early investigation of Claus into 

 the anatomy and development of members of 

 the Cyclopidae it was concluded that the two 

 pairs of appendages following the first maxillae ^ 



are not two pairs but the inner and outer 



divisions of a single pair. It was pointed out FlQ - 259. Mouth parts of Cy- 

 , -, i , . , . . , r\ i ^i clops. Kf second maxilla ; 



that though in the majority of Copepods they K f maxilliped (first tho- 



are inserted separately on the body, in Cyclops 

 their bases are actually united (Fig. 259). This 



racic appendage) ; M man- 

 dible ; MX first maxilla 

 (after Claus). 



