WORMS. 153 



138. The young come from eggs as free-swimming ani- 

 mals, furnished with eyes and jointed appendages provided 

 with hairs. In this condition they swim about for a while, 

 and then acquire a bivalve shell, and in this state remotely 

 resemble the little creatures shown in Fig. 138. 



In Fig. Ill, the letter A represents its first appearance 

 from the egg. Its next appearance with the bivalve shell is 

 shown at B. After remaining a free-swimming animal for a 

 while it becomes attached to the rock, adhering by means of 

 appendages on the head, and, then moulting, it loses its former 

 appearance ; the appendages change their proportions, new 

 parts are added, the eyes disappear, a limy shell gradually 

 forms around it, and it assumes characters entirely unlike 

 those of its earlier stages. 



CHAPTER XX. 



WORMS. 



139. AMONG the animals thus far studied, having a body 

 composed of segments, the pupils have seen that in all cases 

 the appendages were jointed, that is, the legs and antennae 

 were composed of distinct segments or joints ; and, with the 

 exception of the myriapods, or centipedes, the animals pos- 

 sessed a limited number of segments to the body. 



In the group of animals now to be studied the worms- 

 the body has, generally speaking, an indefinite number of 

 segments, and there are no jointed appendages attached to it. 



