86 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



simple mouth surrounded with ciliated tentacles. The 

 colony often takes a plantlike form ; sometimes spreads, 

 like fairy chains or lacework, over other bodies ; or 

 covers rocks and seaweeds in patches with a delicate 

 film. The majority secrete carbonate of lime. A poly- 

 zoan shows its superiority to the coral, which it resem- 

 bles, in possessing a distinct alimentary canal and a 

 nervous system. The cells of a group are never con- 



FIG. 42. Polyzoans : i. Hornera lichenoides, natural size. 2. Branch of the same; 

 magnified. 3. Discopora Skenei, greatly enlarged. 



nected by a common tube, as in coelenterates. There 

 are both marine and fresh-water species. 



The Brachiopoda or "lamp shells" have a bivalve 

 shell, the valves being applied to the dorsal and ventral 

 sides of the body. The valves are unequal, the ventral 

 being usually larger, and more convex ; but they are 

 symmetrical, i.e., a vertical line let fall from the hinge 

 divides the shell into two equal parts. The ventral 

 valve has, in the great majority, a prominent beak, per- 

 forated by a foramen, or hole, through which a fleshy 

 stalk protrudes to attach the animal to submarine rocks. 



