ARTICULATA. 103 



twenty -one, of wliicii seven belong to the Lead, trunk, and tail, respectively, 

 and when a smaller number appears, it is generally to be attributed to the 

 fusion of several into one. In Jig. 22, pi. 78, the head is observed to be 

 distinct, whilst mfig. 1 it forms a single piece with the trunk, the tail being 

 articulated and turned beneath, although partly visible in Jigs. 3 and 8. In 

 Limulus {Jig. 34), the segments of the tail are fused into a single shelly 

 plate, as well as those of the head and trunk or cephalo-thorax, the shield or 

 plate of which is called the carapace. 



The moulting of the larger Crustacea takes place about once a year, but 

 Mr. Lyell mentions a species of Cancer covered with oysters six years old, 

 so that the shell could not ha^•e been renewed during this period. Young 

 and growing animals moult frequently, as in Daphnia, which may change 

 every two days. 1 'revious to moulting, a crustacean becomes sickly and 

 refuses food, the carapace becomes loosened, and the corium beneain 

 secretes a new, soft, and membranous shell, which finally becomes calcareous. 

 When the old shell has become loosened, tlie animal, after various exertions, 

 extricates itself from it in about half an hour, withdrawing the antenna, 

 feet, and even the gills, from an external film. The shell of the limbs splits 

 to allow the large extremity to pass, and it sometimes happens that a limb 

 is left in the old shell. Some of this class have the power of throwing ofi" a 

 limb. 



Tlie Crustacea feed mostly upon animal food, which is taken sometimes 

 solid and sometimes in a liquid state, and the oral organs are modified 

 accordingly. Some small species are very destructive to the timber of ships 

 and docks. 



In forms like the lobster, swimming is effected by striking downwards 

 and inwards with the tail, which forces the body in a backward direction. 

 In the hrancliiopoda the feet are used in swimming, not being adapted to 

 walking; and in the entomostraca {Jigs. 27, 29), the body is impelled 

 through the water by the feet, in a succession of jerks. 



The walking feet of the Crustacea generally amount to ten or fourteen. 

 In some {fig. 1, &c.), the anterior pair are robust, the extremity provided 

 with a movable thumb forming pincers. Sometimes the end of a foot 

 can bend, so as to form a hook ; and in other cases the last joint closes upon 

 an extension or enlargement of the preceding joint, forming an organ of 

 prehension. 



There are two mandibles, four lower jaws, succeeded by six auxiliary or 

 foot-jaws, followed by the feet ; and when there are fourteen feet, the four 

 anterior ones are transformed from the four posterior foot-jaws. 



Most of the Crustacea see well, and the eyes are either sessile or placed 

 upon pedicles. Some of the parasitic species are without eyes in their 

 perfect and affixed state, although provided with them when young, and 

 capable of swimming about. As in the insects, both compound eyes and 

 stemmata are found in this class, but the former are the most common. In 

 a few cases both are found together. 



There are sufficient reasons to believe that the sense of smelling is present 

 in the Crustacea, although its special organ has not been discovered. 



307 



