97 



CHAPTER V. 



PASSIVE ORGANS OP LOCOMOTION, CONTINUED. OF HONE. 



THE distinction of animal textures into hard and soft prevails 

 very extensively throughout the animal series. The former are 

 characterized by containing a proportion of inorganic material, in 

 combination with animal matter, sufficient to give them that degree 

 of hardness which is their principal physical property. 



Among the invertebrated classes there are hard parts, although 

 very differently constituted from those of the higher animals. They 

 serve an analogous purpose, being a basis of support for the soft 

 parts, and in many instances a protection to them, and affording a 

 surface of attachment for the muscles of the animal; thus playing 

 an important part in its locomotion, or in its ordinary movements. 

 To this category we may refer the earthy support to the soft fleshy 

 mass, whether as an internal stem or axis, or as an external cover- 

 ing, which is to be found among the polypifera, performing a func- 

 tion similar to the skeleton of the higher animals, and composed of 

 carbonate of lime, with a little phosphate, in combination with a 

 small quantity of animal matter. 



The calcareous plates of the star-fish and sea-urchin (asterias and 

 echinus), the hard coriaceous covering of insects, the hard external 

 integuments of Crustacea, and the infinitely various shells of the 

 gasteropoda and conchifera, must all be regarded in the light of hard 

 parts performing the offices above referred to. 



The skeleton of the higher animals is internal ; it is clothed by 

 the muscles and other soft parts. The first example of this arrange- 

 ment is met with in the cephalopodous mollusks, in which certain 

 cartilaginous plates are enclosed in the body of the animal, protect- 

 ing certain parts of the nervous system. The skeleton of the lowest 

 organized fishes, although much more extensive, and of a more com- 

 plicated arrangement, is yet placed but little above that of those 

 animals. It is composed of a kind of cartilage, which in its greater 

 density, and in its having a certain quantity of calcareous deposit 

 around it, approaches the nature of the skeleton of the higher 

 classes. 



Bone is the substance employed to form the internal skeleton of 

 the osseous fishes, of reptiles, birds, and mammalia. It forms organs 



VOL. I. H 



