CRUSTACEA CONSIDERED AS A CLASS. 353 



and composed of a single cavity. The respiratory organs 

 consist of branchiae or gills (Fig. 479), as already stated. 



The voluntary muscles are composed of colorless fibres, 

 inserted on the interior of the solid covering or crust, 

 which may be called an external skeleton. 



The nervous system consists of a double series of gan- 

 glions, situated on the ventral side of the body, and 

 connected with the cephalic ganglions situated before and 

 above the gullet. 



The sense of sight is present in nearly all ; that of 

 hearing is indicated by organs only in the highest. Little 

 is known in regard to their sense of smell or taste. 

 Their sense of touch is highly developed. 



The Crustaceans have a wonderful power of repairing 

 injuries to themselves. If a leg, or other appendage, be 

 broken off, another like it soon grows in its place ! 



But one of the most remarkable facts regarding the 

 Crustaceans is that from time to time they shed the shell 

 in one piece, so that the cast-off shell looks exactly like 

 the perfect animal antennae, eyes, jaws, legs, paddles, 

 and even every hair, are all just as they were when they 

 covered the living crustacean ! The animal comes out of 

 its shell through a rent on the back, and is at first very 

 soft ; it at once increases in size, and in a few days its 

 skin becomes as hard as the shell which it cast off'. 

 This shedding of the shell is necessary for the growth 



i^J ' 



of these animals; for while the shell remains the Crusta- 

 cean can grow only just large enough to completely 

 fill it. 



When a Lobster is ready to shed its shell, there are two hard 

 stone-like bodies at the sides of the stomach, and it is supposed that 

 these furnish a part of the solid matter for the new shell ; for they 

 immediately begin to grow smaller after the moulting, and soon en- 

 tirely disappear. 



