ELEMENTS 



OF 



NATURAL HISTORY. 



INVERTEBRAL ANIMALS. 



(INVER TEBRA TA.) 



THE animals destitute of a vertebral column and bony skele- 

 ton 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 subse- 

 quent 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 verte- 

 bral animals, some are found with the body unprotected, ex- 

 cept by a soft skin ; others are covered by a shell ; while others 

 have their members enveloped in crustaceous plates. The cir- 

 culating 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 nervous threads. No class of inverte- 

 bral 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 by a process analogous to the budding of vege- 

 tables. 



VOL. II. A 



