ay’ * >. 
ARTICULATIONS OF THE BONES. 12) 
In taking a rapid sketch of the different parts of 
the skeleton, we may perceive, 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, differmg from bone in compo- 
sition, and destitute of articulation. These circumstances 
haye 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, 1s claimed by M. Lamanrk, in his 
‘© Systéme des Animaux sans Vertebres,” Svo, 1801. The 
same groups, however, have been known to naturalists since 
the days of ArisrorLe, the vertebral having been termed 
Sanguineous, and the invertebral Kxangwincous. Among 
the vertebral animals, are included Mammalia, Birds, Rep- 
tiles, and Fishes ; and, among the imvertebral, those which 
were termed by Linnevs, Insects and Worms. 
This primary division of animals inte 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- 
portion of animal matter. In some cases, there are traces 
