STRUCTURE OP THE CRUSTACEA. 75 



.hat distributes it to the whole body. On its return from the 

 vessels of the body, it is collected into another vessel situated 

 near the back, and performing in some measure the office of a 

 ventricle, and is again sent to the gills. Their nervous sys- 

 tem, and the degree of sensation they enjoy, are not essentially 

 different from those of insects. 



They are covered by a pretty thick, firm shell, which en- 

 velops them completely. This serves for a shelter and pro- 

 tection to their soft parts, and also answers to them the same 

 purpose, as an instrument of motion, that the internal system 

 of bones does to the vertebral animals. As this shell is inca- 

 pable of growth, it is occasionally changed, to make room for 

 the constant increase in size of the animal. It is thrown 

 off, and their bodies remain for a time entirely naked, and 

 exposed in a soft and defenceless state. In this case the ani- 

 mal generally retires to some place of concealment and secu- 

 rity, and remains till the shell is restored. This is done by 

 the deposition of calcareous matter on the external membrane 

 of the skin, which consequently becomes hard and firm, and 

 finally takes the place of the old shell. 



The Crustacea have always as many as six claws, and fre- 

 quently more. The two anterior ones are often prolonged, 

 enlarged, and armed with teeth, so as in some measure to act 

 in assisting the jaws. Their antennre, as those of insects, are 

 probably intended to serve as very delicate organs of touch. 

 They possess the sense of smelling, but naturalists have not 

 been able to satisfy themselves in what organ it resides. 

 The organ of hearing has been discovered. Their eyes are 

 not placed loosely in a socket, but are fixed and immovable; 

 and, to remedy the inconvenience which would result from this 

 arrangement, they are, in some species, situated upon the end 

 of a pedicle or stalk, which is capable of motion in every 

 direction. 



The stomach of some of the Crustacea presents a very sin- 

 gular and remarkable structure. It is exemplified particularly 

 in the crab, lobster, crawfish, and others of the same kind ; 

 and is found in no other animals of any class. Near the lower 

 end of the stomach, where it begins to grow narrow, are sit- 

 uated a number of teeth, or substances of a bony nature re- 

 sembling teeth, generally five in number. They are placed 

 upon the opposite sides of the organ, and, being moved by 

 muscles belonging to them, they grind up thoroughly the food 

 passed between them, which then goes out ai the orifice into 

 the intestines 



