126 CLASSIFICATION OF THE ANIMAL KINGDOM. 



it into any convenient empty shell. In their structure they are analogous 

 to the Lobster, though widely different in shape. Their chelae or claws 

 are of different sizes; the two succeeding pairs, unlike those of the Lobster, 

 are very strong, and, instead of forceps, terminate in a sharp point, to 

 enable the animal to move along; and their tails, instead of being provided 

 with lamellae for swimming, are transformed into a sort of leg-like appen- 

 dages, to enable them to hold on to the inner part of the shells which 

 they inhabit. 



The Bmchyura have much more concentrated skeletons, their tails are 

 very short, they have powerful chela), and their legs are adapted for walking 

 on land. At certain times of the year crustaceans change their skin, or 

 rather shell; they cast off every part down to the very joints in a 

 remarkable way, and a new one comes in its place. The alimentary canal 

 of these animals is very simple, and consists of an oesophagus, a stomach, 

 in which fs contained a singular masticating apparatus, and a straight 

 intestinal tube, it is also provided with a liver and biliary ducts. Their 

 circulatory system consists of branchiae, variously disposed, in the lower 

 orders the legs used in swimming have certain fringed lamellae appended 

 to them, which exercise this function; in the higher orders these lamellae 

 are attached to the tail and sides of the body, near the origin of the 

 legs. The heart of the lower orders of this class is a dorsal vessel, similar 

 to that of insects, but in the decapod division it becomes more centralized. 

 It is possessed of various large arteries, which disperse the blood through 

 the body, and the venous apparatus analagous to that of spiders, consists 

 of delicate sinuses or cavities which freely communicate with each other. 

 The nervous system of this class passes through all the gradations of 

 development met with throughout the Homogangliate division of the animal 

 kingdom. The lowest types indeed of the Crustacea are actually less highly 

 organized in this respect than the humblest Annelidans, for, in the latter, 

 we always find the two lateral masses of the supra-cesophagal ganglion 

 united, but in some Crustacea, as Talitrus, this lateral division is perfectly 

 evident. As we rise higher, however, we find a gradual concentration of 

 the nervous chain, until at last, in the Common Crab, the whole is gathered 

 into one mass or brain, from which radiations are thrown out to all parts 

 of the body; a type of development equal to, but not higher than, the 

 Spider. 



Allusion has before been made to a division of the ganglionic chain in 

 the class Insecta, similar to that found in vertebrate animals; and the 

 same character is met with in the class we are now considering. Each 

 ganglion consists of two portions, and it is supposed that the inferior is 

 connected with sensation, and the superior with the movements of the 

 body; which arrangement, however, is precisely opposite to that met with 



