ARTICULATIONS OF THE UONES. 



111 taking a rapid sketch of the different parts of 

 the skeleton, we may i>erceive, in the more perfect ani- 

 mals, that, while some bones, such as those of the head and 



the vertebra?, are always present, others, as the sternum, 

 ribs, and extremities, are only found in the species of parti- 

 cular classes or orders. The animals in which a skeleton 

 is found, even in its simplest forms, are few in number, in 

 comparison with those whose hard parts are either connect- 

 ed with the integuments, as we have already noticed, or 

 form solid pieces of support, differing from bone in compo- 

 sition, and destitute of articulation. These circumstances 

 have given rise to a division of animals into two classes, the 

 Vertebral and Invertebral ; the former possessing a verte- 

 bral column, of which the latter are destitute. The honour 

 of forming these divisions, is claimed by M. LAM ARK, in his 

 " Sy steme des Animaux sans Vertebres," 8vo, 1801. The 

 same groups, however, have been known to naturalists since 

 the days of ARISTOTLE, the vertebral having been termed 

 Sanguineous , and the invertebral Exangmneous. Among 

 the vertebral animals, are included Mammalia, Birds, Rep- 

 tiles, and Fishes ; and, among the invertebral, those which 

 were termed by LINNEUS, Insects and Worms. 



This primary division of animals into vertebral and inver- 

 tebral, is natural ; and the distinction, after a little acquaint- 

 ance with the species, is obvious, and of easy application. 



It may, however, be observed, that, among the inverte- 

 bral animals, there are hard parts, which, though differing 

 from bone in composition and structure, answer the same 

 purposes in the animal economy. They all appear to be 

 formed after the manner of shell, or horn, by the addition 

 of layers of earthy matter on one of their surfaces, and not 

 by the extension of a cartilaginous frame-work. In compo- 

 sition they likewise bear a close resemblance to shell, con- 

 sisting principally of carbonate of lime, with a variable pro- 

 ]>ortion of animal matter. In some cases, there are traces 



