752 
INVERTEBRAL ANIMALS. 
Tue animals destitute of a vertebral column and bony skeleton, form the 
second, and, by far, the most numerous, group of living beings. in the system 
of Linnzus, the invertebral animals were included in two great classes, 
Insecta and Vermes. But subsequent investigations into their nature and 
organization, have given rise to more numerous and better characterized 
groups. Possessing little analogy in point of structure with the vertebral 
animals, some are found with the body unprotected, except by a soft skin; 
others are covered by a shell; while others have their members enveloped 
in crustaceous plates. The circulating system in this division is also less 
perfect than in the vertebral animals; and, with the exception of a few 
groups, none have red blood. The nervous system appears also in a less 
complete form ; and, instead of the medullary mass of the brain and spinal 
chord of the higher classes, they present only ganglions or knots in the ner- 
vous thread. No class of invertebral animals possesses all the organs of 
sense; for while some are destitute of the organs of hearing, others seem 
deprived of the faculty of smell and sight, and many appear to be guided 
only by the sense of touch. The sexes, besides, are in many groups united 
in the same individuals, and in others the species is continued in a process 
analogous to the budding of vegetables. 
The animals of this division are but feebly endowed with the functions 
of relation. Many of them, indeed, almost deprived of locomotion, or 
fixed to other bodies, have neither choice of situation or food, but remain 
for the term of life in the places where they originally had their birth. But 
the want of intelligence is largely made up to many classes of this division, 
by their superior instinctive powers, which, in as far as regard their subsis- 
tence and reproduction, surpass that of the vertebral animals. In one 
very large class, the insects, this instinctive intelligence is displayed in a 
very striking manner, in the combination of individuals for one common pur- 
pose, and in the wonderful subsidiary arrangements of their commonwealths. 
[t has been observed, as a distinction between the vertebral and the inverte- 
bral animals,-that while in the former, the bones or hard parts are more o. 
less formed of phosphate of lime; the hard parts of the latter, such as the 
shells of the mollusca and crustacea, and the stony matter of corals and 
madrepores, are chiefly composed of carbonate of lime. The invertebral 
animals, as noticed in the introduction, are arranged by Cuvier into three 
great divisions. 1. Those which have no skeleton; in which the muscles 
are attached only to the skin, which proves a soft contractile covering, in 
