AND PHYSIOLOGY, 



105 



Fig. 118. 



Microscopic view of Shell of Pinna. 



is calcareous, horny, or leathery. This skeleton, called a 

 polypary, or in popular language coral, is secreted by the 

 skin of the animal, and seems to be truly an internal skele- 

 ton. It contains, say ninety-five per cent, of carbonate of 

 lime, some two or three per cent, of fluorides and phosphates, 

 and from four to eight per cent, of organic matter. 



The Acalephse, another class of Radiates, are mostly gela- 

 tinous ; but a few are cartilaginous, and some have a nuclear 

 skeleton. The Echinodermata, with few exceptions, have an 



Fig. 119. 



Microscopic section of hinge of Mya showing a crystalline structure. 



What is coral ? What is the skeleton of other Radiates J 



