AND PHYSIOLOGY. 



103 



on the jaws, tongue, and palate. They are repeatedly re- 

 newed during the life of the animal. Fig. 116 represents 

 the microscopic structure of the tooth of an Eagle Ray. 



Fig. 116. 



Microscopic structure of Tooth of Eagle Ray. 



223. Skeleton, or Harder Parts of Invert ebrata— The 

 harder parts of invertebrate animals generally differ so much 

 from bones in composition and^ structure that the term Osteo- 

 logy may perhaps be less proper than that of Skeleton in 

 describing them. 



224. Skeletons of Crustaceans— Among the Articu- 

 lata the Crustaceans have a more or less solid external crust 

 of carbonate of lime in a base of peculiar azotic matter, in- 

 soluble in caustic potash, called Chitine. 



225. Skeletons of Insects . — Insects, the most numerous 

 class of articulated animals, have a chitinous covering, gener- 

 ally leathery, but sometimes solid. The Arachnoidea have a 

 similar skin, soft or coriaceous, rarely horny ; and the Anne- 

 lida a very thin epidermis. The Helminthes, Rotatoria, and 

 Turbellaria, reckoned in this branch of the animal kingdom 



What is curious about their renewal? 224. Describe the skeleton of Crustaceans! 

 225. What is the skeleton of Insects f 



5* 



