THE CRUSTACEANS 
TINHESE are the crabs, lobsters, sand-fleas, barnacles and a host of 
minute creatures of both 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 breathe through internal tubes called trachew 
live on land. A further difference between crustaceans and insects 
is that in the former the legs are bifurcated while in insects they are 
simple and linear. 
In crustaceans and insects the body is made up of distinct 
segments separated by constrictions. This is also true of the higher 
worms but while the legs of crustaceans and insects are jointed, 
those of worms are usually mere stump-like flappers. Moreover, in 
worms the body is made 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 off one from another by sharp 
constrictions. In crustaceans, however, we find that there is no 
sharp line of demarkation between the head and thorax, and only the 
abdomen is more or less sharply 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 and insects are probably descended from 
worm-like ancestors but while crustaceans have been developed 
mainly for life in the water, insects have become more complex and 
live mainly on land. 
The body covering of a crustacean 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 
