752 



INVERTEBRAL ANIMALS. 



The 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 Linnaeus, 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 liltle analogy in point of structure with the vertebral 

 animals, some are found wilh 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. 

 It has been observed, as a distinction between the vertebral and the inverte- 

 hx-dl animals, that while in the former, the bones or hard parts are more or 

 less formed of phosphate of lime ; the hard parts of the latter, such as the 

 shells of the moUusca and Crustacea, and the stony matter of corals and 

 STiadrepores, 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 



