CRUSTACEA. 131 



■segments of the thorax carry five pairs of walking-legs, hence 

 the name Decapoda applied to the order. Of these legs, the 

 first three pairs have their extremities converted into nipping- 

 claws or " chelae," and the first pair is much larger than the 

 others, and constitutes the weU-known great claws of the lob- 

 ster. The last two pairs of legs simply terminate in pointed 

 extremities, and not in pincers. The segments of the abdo- 

 men, with the exception of the hindmost, carry each a pair of 

 paddle-like appendages, which are used in swimming, and are 

 called the " swimmerets." The last pair of swimmerets are 

 attached to the last segment but one, and are very greatly ex- 

 panded, so as to form a very powerful tail-fin. The last seg- 

 ment of all is known as the " telson," and is not provided with 

 any lateral appendages. 



The mouth in the lobster leads by a short gullet into a 

 globular stomach, which is furnished with a calcareous appa- 

 ratus for grinding down the food, commonly called the "lady 

 in the lobster." The intestine is continued backward from 

 the stomach without convolutions, and opens by a distinct 

 anus placed in front of the telson. A well-developed liver is 

 also present. The heart is placed dorsally, and is filled with 

 aerated blood derived from the gills, which it propels through 

 every part of the body. The gills, or branchiee, are pyrami- 

 dal bodies attached to the bases of the legs, and placed in 

 a kind of chamber formed beneath the great shield, or cara- 

 pace, on each side of the body. They consist each of a central 

 stem supporting numerous lateral branches, and they are richly 

 suppUed with blood. The water which fills the gill-chambers 

 is constantly renewed by the movements of the legs, and thus 

 the gills are kept constantly supplied with fresh water. The 

 nervous system is placed along the ventral surface of the body, 

 and has its usual form. The organs of sense are the two pairs 

 of feelers or antenna, the compound eyes, and two organs of 

 hearing. 



B. Anomura. — The most familiar members of this tribe 

 are the hermit-crabs {Paguridm) which occur so commonly on 

 every shore. They are distinguished by the fact that the ab- 

 domen is quite soft, and is not protected by a chitinous crust. 

 The animal, therefore, is compelled to protect the defenceless 

 part of the body in some artificial manner, and this it eifects 

 by appropriating the empty shell of some dead moUusk, such 

 as the common periwinkle or whelk. The abdomen is pro- 

 vided with special appendages to enable the intruder to re- 

 tain firm hold of his borrowed dwelling, at the same time that 



