64 



THE CHAIN OF LIFE. 



the earliest of which are shown in Fig. 55. The modern Sin- 

 gula is protected by a dcH'^ate two-valved shell, composed, 

 unlike that of most other mollusks, of phosphate of lime or 

 bone earth. It lives on sand-banks, attached by its long 

 flexible stalk, which it buries like a root in the bottom. Its 

 food consists of microscopic organisms, drifted to its mouth 

 by cilia placed on two arm-like processes, from which the group 

 derives its name. In the modern world about one hundred 



Fig. 52. — Exiracrinus Briareus. 

 Reduced. Jurassic. 



Fig. S3 —Pentacrinus caput-medusce. 

 Reduced. Modern. 



species of Brachiopods are known, belonging to about twenty 

 genera, some of which differ considerably from the Lingulae. 

 The genus Terebratula^ represented at Fig. 5 5 a, is one of the 

 most common modern as well as fossil forms,' and has the 

 valves unequal, with a round opening in one of them for the 

 .stalk, which is attached to some hard object, and there is an 

 internal shelly loop for supporting the arms. 



hi- 



