CRUSTACEANS. 599 
(segments) and in the joints alone is the integument thinner and 
more flexible. 
Let us first consider the external structure of these animals 
a little more closely. We have seen above that the body of insects, 
the myriapods excepted, is divided into head, trunk and abdomen. 
Such a division as this does not occur in the crustaceans. In many 
the head is not distinct from the thorax, and its place is indicated 
solely by the position of the eyes, the attachment of the antenne 
and the presence of the mouth; it is most completely connected 
and, as it were, intimately fused with a large part of the body, in 
which the principal viscera are contained, and which may be com- 
pared not with the thorax alone of insects, but also with the an- 
terior portion of their abdomen. To this part of the body another 
succeeds, in which only the posterior portion of the intestinal canal 
and of the nervous system is contained, and which is commonly 
called the tail; thus it is for instance in the cray-fishes. In 
other instances the hindmost portion is less obviously separated 
from the trunk, and the entire body is parted into rings or segments. 
In the X¢phisoura the divisions or segments are not recognisable on 
the dorsal surface, for the body is parted into two shields alone, to 
which a long and pointed appendage is attached posteriorly. In 
others the cephalothorax is more or less distinct from the rest of 
the body, which is not obviously divided into segments but is 
covered by a bivalve, membranaceo-horny shell (Cypris). In the 
Cirripedia the partition into segments ceases; the body ends with 
a thin tail without appendage. They are surrounded by a mantle, 
in which, as in the class of the mollusca, calcareous plates are formed 
which resemble the shells of these animals. 
To the anterior portion of the cephalothorax, or to the head 
itself, whenever it is distinct, the antenne, eyes and oral organs are 
attached. When there are four antenne, as is the case with most, 
they are placed either in the same plane, or the one pair is placed 
above the other, so that according to their position they may be 
distinguished as middle and external, or as upper and lower. The 
external or lateral antenne are implanted close to the eyes, some- 
times beneath the eyes. They consist commonly of three or four 
larger and thicker joints at the base, and a filiform part, terminating 
finely and composed of numerous joints. Sometimes the antenne 
terminate in two or three filaments of this kind. In some the 
