ANIMAL ORGANIZATION. Ill 



the Phosphate of lime is employed for forming 

 these hard and unyielding structures ; and often 

 both these calcareous substances are united to- 

 gether in different proportions in the same solid 

 fabric. When the carbonate of lime predomi- 

 nates, or is the sole earthy ingredient, it consti- 

 tutes Shell : when there is a greater proportion of 

 the phosphate, it is called a Crust, as is the 

 case with the coverings of the lobster and the 

 crab : when the earthy matter consists almost 

 wholly of phosphate of lime, it composes the dif- 

 ferent forms of Bone. I shall have occasion to 

 describe the formation and properties of each of 

 these structures in the sequel. 



The protection of the delicate structure of the 

 fabric from the injurious influence of external 

 agents is an object of great importance in the 

 animal economy, and is one which nature has 

 shown extreme solicitude to secure. For this 

 purpose she has provided the integuments, under 

 which designation are included not merely the 

 skin, but also all the parts that are immediately 

 connected with it, and are formed and nourished 

 by the same vessels. No parts of the animal 

 structure present greater diversity in their form 

 and outward appearance than the integuments ; 

 yet it is easy to discover, amidst all these va- 

 rieties, that the same general plan has been fol- 

 lowed in their construction, and that each par- 



