^8 THE LIVING WORLD. 



Fig. 8, g, is an example, the group of hydroids 

 including corals, jelly-fishes and sea anemones. In 

 nearly all other lines of descent from the Gastraea 

 there was soon acquired a new fundamental feature. 

 The mouth, which originally served for the entrance 

 of food and the excretion of waste matter as well, 

 was replaced by a pair of openings, one serving for 

 each function (Fig. 8, b). Just how this took place 

 is perhaps still a little problematical. The proba- 

 bility seems to be that the one opening of the Gastraea 

 elongated into a slit and then closed in its middle. 

 The two ends of this elongated slit remained open, 

 however, one for the entrance of food, and the other 

 for the exit of refuse. With the subsequent elonga- 

 tion of the body these two openings became widely 

 separated from each other. After this further 

 modifications of the body arose. Developing a shell 

 on its back (Fig. 8, e), it started along a line which 

 has produced the type which we have called mol- 

 lusks (snails, oysters). Along one line of descent 

 this shell assumed a spiral twist, giving rise to the 

 snails (Fig. 8, /). In another the shell became 

 divided into two pieces, one on either side, giving 

 rise to the clams, oysters, etc. ( ' Lamellibranchiata). 

 In another line it became elongated and divided into 

 segments, and this gave rise to the large type of 

 animals known as segmented animals (Fig. 8, d\ 

 (segmented worms, Crustacea, Insectd). An elonga- 

 tion and segmentation and subsequent modification 

 in many important particulars, the chief of which 

 was the development of an internal skeleton, pro- 

 duced the type of Vertebrata. The details of these 



