THE CRUSTACEANS 



^pHESE are tlie crabs, li>l)sters, sand-fleas, barnacles and a host of 



minute creatiires of botli salt and fresh water. They breathe by 

 means of plume-like gills which are usually attached to the bases 

 of the legs, and thus the vast majority of crustaceans live in water, 

 whereas insects which brcatlie through internal tubes called bxirhcd' 

 live on laud. A further dilfereuce between crustaceans and insects 

 is that in the former the legs are l)ifurcated while in insects they are 

 simple and linear. 



In crustaceans and insects the bod}' is made up of distinct 

 segments separated by constrictions. This is also true of the higher 

 worms l)ut while the legs of crustaceans and insects are jointed, 

 those of worms are usually mere stump-like flappers. ]\Ioreover, in 

 worms the bodj- is macfe up of segments which are similar each 

 to each, whereas in crustaceans and insects the body is composed of 

 dissimilar segments. 



In insects we find three distinct regions called the head, thorax 

 and abdomen, and these are marked olt one from another by sharp 

 constrictions. In crustaceans, however, we find that there is no 

 shar|i line of demarkation between the head and thorax, and only the 

 abdomen is more or less sharpl_v defined from the rest of the body. 

 In worms, crustaceans and insects the brain lies in the head above 

 the intestine, and two cords of nerve tissue extend downward from 

 the brain on both sides of the throat and connect the brain with 

 the main line of nerve fibres which extend in a double line down 

 the middle of the lower side of the animal. 



Both crustaceans aud insects are proljably descended from 

 worm-like ancestors Init while crustaceans have been devehiped 

 mainly fi'r life in the water, iusei'ts have become more complex and 

 live mainly on laud. 



The body covering of a criistacean is tough, and rendered still 

 harder by deposits of carbonate of lime, so that the animal is en- 

 closed, so to speak, within its own skeleton. At the joints, however, 

 the skin is flexible, allowing a certain freedom of movement. This 



