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 

 i 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 

 ;he head is not distinct from the thorax, and its place is indicated 

 solely by the position of the eyes, the attachment of the antennae 

 md the presence of the mouth ; it is most completely connected 

 jind, as it were, intimately fused with a large part of the body, in 

 vhich the principal viscera are contained, and which may be corn- 

 Dared not with the thorax alone of insects, but also with the an- 

 ;erior portion of their abdomen. To this part of the body another 

 [succeeds, in which only the posterior portion of the intestinal canal 

 ind of the nervous system is contained, and which is commonly 

 ;alled the tail; thus it is for instance in the cray-fishes. In 

 pther instances the hindmost portion is less obviously separated 

 |rom the trunk, and the entire body is parted into rings or segments. 

 n the Xiphisoura the divisions or segments are not recognisable on 

 he dorsal surface, for the body is parted into two shields alone, to 

 vhich a long and pointed appendage is attached posteriorly. In 

 thers the cephalothorax is more or less distinct from the rest of 

 lie body, which is not obviously divided into segments but is 

 overed by a bivalve, membranaceo-homy shell (Cypris}. In the 

 wrripedia the partition into segments ceases ; the body ends with 

 thin tail without appendage. They are surrounded by a mantle, 

 n which, as in the class of the mollusca, calcareous plates are formed 

 diich resemble the shells of these animals. 



To the anterior portion of the cephalothorax, or to the head 

 tself, whenever it is distinct, the antenna, eyes and oral organs are 

 Attached. When there are four antennas, as is the case with most, 

 bey are placed either in the same plane, or the one pair is placed 

 bove the other, so that according to their position they may be 

 istinguished as middle and external, or as upper and lower. The 

 xternal or lateral antennas are implanted close to the eyes, some- 

 imes beneath the eyes. They consist commonly of three or four 

 arger and thicker joints at the base, and a filiform part, terminating 

 inely and composed of numerous joints. Sometimes the antennas 

 erminate in two or three filaments of this kind. In some the 



