M.T. Thorell on the Terminology of the Argulide. 449 
Neither am I acquainted with any transition between the Peeci- 
lostoma and Siphonostoma. It is freely granted that, in such 
forms as lack the appendages of the mouth, it may be sometimes 
difficult to determine to which series they should be referred ; 
but in such instances correspondences in other parts of the ge- 
neral structure must decide the question: for example, it is 
easy to see that Monstrilla is a Poecilostome and approaches the 
Coryceeidee, to which family it is, indeed, referred by Claus. 
From what has now been said, it follows that I cannot admit 
that my arrangement of the Copepoda renders difficult the forma- 
tion of subordinate groups, families, and genera. On the other 
hand, it is conceded that it sometimes removes from each other 
forms which in habitus stand tolerably near together; but this 
inconvenience is in a great measure compensated by the paral- 
lelism of the series. Certain it is that by the division of the 
Copepoda into Copepoda carcinoidea and C. parasitica, adopted 
by Claus, this inconvenience is not avoided : any definite limita- 
tion of these two groups based upon characters drawn from the 
form is not to be thought of. Claus himself admits this, but 
consoles himself with the consideration that the impossibility of 
a sharp definition of limits lies in the very nature of any system 
which would be true to nature. Many, however, will be found 
who will agree with me in not resting content with such reason- 
ing, but in regarding fixed principles for the forming of divisions 
as necessary for any systematic arrangement. And if the source 
of such division be sought in the modifications of organs which 
are constant in their nature and significance throughout the 
entire life of the animal, which has seemed to me to be the case 
with the oral organs, a sharp definition of the limits of groups 
will not necessarily make the system one-sided and unnatural. 
[Note. Ina rather lengthy footnote appended to the preceding 
paper, Prof. Thorell makes the following important remarks re- 
lative to the nomenclature of the various portions of the body 
in the Argulids. Reverting to p. 150, we find that Prof. Thorell 
applies the terms head or head. shield, trunk, and tail to the prin- 
cipal divisions of the body in Argulids, calling the pieces at- 
tached posteriorly to the latter appendages, not postabdomen, In 
connexion with this he says :-— 
“ Such a terminology differs somewhat from the now generally 
received division of the Crustacean body into head, thorax, abdo- 
men, and pustabdomen. ‘There are several objections to this divi- 
sion. Jhorax and abdomen are divisions founded (throughout 
the greater portion of the Articulate series) on notions almost 
exclusively drawn from the class of Insects, and are not. properly 
applicable to any but that class and the Arachnids, where they 
. 
