14 



RECAPITULATION. 



1. All animals are included under the heads vertebrata and in- 

 vertebrata. 



2. Great advantage to be derived from a knowledge of arrange- 

 ment and classification. 



3. The osseous system is neither the most important nor the 

 most uniformly existing. 



4. Skeleton exists in every class of animals, but modified accord- 

 ing to each class. 



5. Skeleton generally internal in the radiata, and external in the 

 articulata, subject to exceptions. 



6. Skeleton remarkable for inconstancy and want of symmetry 

 in the mollusca. 



7. Basis of skeleton — silica in the lowest radiata, carbonate and 

 phosphate of lime in the articulata, and carbonate of lime chiefly in 

 the mollusca. 



8. No bony skeleton where regularly formed brain does not 

 exist. 



CHAPTER III. 



VERTEBRATA. 



General observations. — It will appear hereafter that the principal 

 part of the nervous system consists in a single central mass extended 

 along the back, and composed of a series of distinct portions, each 

 of which, like the cerebral ganglion of one of the sepiae, is indicated 

 by its giving off one or more pairs of nerves. To coincide with such 

 a disposition, the chief portion of the skeleton is formed by a series 

 of osseous rings, which being mutually articulated and collectively 

 forming a closed cavity, compose a series of spinal vertebra? consti- 

 tuting the spinal column — the distinctive character of this division 

 of the animal sphere. The vertebral column, which is the first 

 rudiment of skeleton observed in the human embryo, is also the 

 primary and most essential portion of it in the higher classes of ani- 

 mals generally, and in many cases alone composes nearly the whole 

 of the skeleton. 



Another characteristic of the skeleton in this department of ani- 

 mals is, that it is placed internal to the soft parts, and is not exuviable 

 in a mass, as it was in most of the invertebrate classes. Here 

 the phosphate of lime is the predominant ingredient, and its propor- 

 tion increases as we ascend through the vertebrated classes. 



The appearance of the skeleton is greatly varied by the situation 

 of the ribs ; for instance, in fish and aquatic mammalia the thorax is 

 placed near the anterior part of the column, to allow of the mobility 



