240 MICROSCOPY FOR BEGINNERS. 



each side often being quite short and difficult to see 

 distinctly, while the other two are usually long and con- 

 spicuous. They are all formed of short and well- 

 marked joints, the number varying greatly in the dif- 

 ferent genera, and sometimes in different species of the 

 same genus. 



One or more black or dark red eye-spots are com- 

 monly present. In some the eye is single, and in the 

 centre of the forehead. It may also be slightly movable 

 at the will of its possessor. The young animal, as not 

 rarely happens, may have two distinct eyes, which, as it 

 grows older, become joined into one and covered by the 

 shell. 



The heart is very frequently visible, especially in the 

 shell-bearing forms, being there placed at the back of 

 the body and near the head. It beats rapidly, and ap- 

 parently sends the colorless blood quickly through the 

 system. 



They all increase and multiply through the formation 

 of eggs, which may remain within the shell and there 

 be hatched, or they may be attached to the parents' body 

 in external clusters. In the shell-bearing forms they 

 are passed into a brood cavity at the back between the 

 body and the shell, where they are kept until the young 

 are hatched, when the latter make their escape into the 

 water, and care for themselves. In those without shells 

 the eggs are passed out of the body into one or two 

 small, pear-shaped sacks called external ovaries, where 

 they remain until hatched. In these cases, however, the 



