450 M. T. Thorell on the Terminology of the Argulidse. 



correspond to distinct sections of the bodily functions. This is 

 not the case in either Myriopods or Crustaceans, where, how- 

 ever, the head is distinctly separated from the trunk; and, in 

 Crustaceans at least (as in the Scorpions), the hindmost segment 

 of the trunk is usually distinct, both in form and function, from 

 the others, thus forming a tail or postabdomen so called. With 

 the first three segments of the trunk, corresponding to the tho- 

 rax of insects, this, however, is by no means the case ; conse- 

 quently the term ' thorax ' seems quite inapplicable, since there 

 is really no definite division of the body to apply it to ; and with 

 the rejection of the appellation ' thorax/ the terms ' abdomen ' 

 and ' postabdomen ' also must necessarily be laid aside. 



'' The anterior segments of the trunk, on the contrary, show 

 here, as in the Myriopods, a strong tendency to coalesce with 

 the genuine head, and their extremities are thus most generally 

 metamorphosed into organs used for chewing or holding food, 

 i. e. foot-jaws. Either it is only one such segment which thus 

 loses its own independent character, and becomes tributary to 

 the head (Edriophthalma), or it is two, unless both pairs of foot- 

 jaws belong to the same segment, as in Copepoda and Argulidse, 

 or all three of the so-called thoracic segments (Decapoda). 

 Sometimes these segments, with their appendages, become more 

 or less rudimentary, or would seem to disappear altogether, as 

 in the Branchiopoda. (In the Squillidse all the first five segments 

 of the trunk are subordinate to the head.) In all these cases 

 it seems better to call any anterior division of the body, however 

 formed by such fusion of one, two, or three posterior segments, 

 head. In the Decapoda, where three segments are united with 

 the head, the term cephalothorax is perhaps a suitable one, if not 

 that of cephalocormus, which I would apply to designate the 

 complete fusion of the whole trunk with the head as a single 

 piece. There is, however, even better reason for using ' cephalo- 

 thorax ' in connexion with the Argulids and many of the para- 

 sitic Copepods, as the CaligidiB, where the extremities or '' foot- 

 jaws ^^ which have become coalescent with the head are not ac- 

 cessory oral organs, but genuine fixing- or seizing-organs. It 

 should, however, be observed that every possible transition is 

 found between such fixing-organs and the ordinary foot-jaws, 

 and that only one, or at most two, segments of the trunk can 

 be regarded as entering into the composition of any such cephalo- 

 thorax. 



" Often, indeed, amongst the Copepoda one of the trunk-seg- 

 ments is united with the head and the foregoing ones ; but in 

 this case the extremities of the segment are always of the same 

 form as those of the preceding trunk-segments, i. e. swimming- 

 feet, not foot-jaws, and such a segment belongs consequently to 



