450  M.T.Thorell on the Terminology of the Argulide. 
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 ¢ai/ 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, 
1.e. foot-jaws. Hither 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 Argulide, 
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 Squillidee 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. Caligide, 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, 1. €. swimming- 
feet, not foot-jaws, and such a segment belongs consequently to 
