ON THE LAST FOUR CLASSES 



OF THE 



ANIMAL KINGDOM 



IN GENEEAL. 



We have already seen (Vol. i. p. 32) that modern Zoologists, 

 in imitation of Lamarck and Cuvier, unite the Fishes, Eeptiles, 

 Birds and Mammals, under the common name of vertebrate ani- 

 mals, to form a large division of the animal kingdom. In these 

 animals tlie main trunk of the nervous system, the spinal cord and 

 the brain, is inclosed in a bony or cartilaginous cavity, which is 

 usually formed of distinct rings or vertebrse. 



The muscles are almost all inserted into internal supports, 

 which, together with the hard investiture of the brain and spinal 

 cord, form what is called the frame-work or skeleton. The body 

 has in general a symmetrical form on tlie two sides, and is divided 

 by an imaginary axis into a right and left half, in which especially 

 the organs of animal life, the nerves and muscles, are correspond- 

 ingly formed and placed. There are never more than four limbs 

 present in these animals; some have only one pair of limbs; in 

 others the limbs are entirely absent. Their blood is red^ The 

 sexes are always distinct, so that bisexual individuals occur as 

 pathological exceptions alone. 



There are always two jaws, one situated above the other, of 

 which the lower especially is moveable. The motion of these parts 

 occurs vertically and not laterally, as is the case in the Articu- 

 late Animals. Ordinarily the jaws are armed with teeth; in the 



1 In AmpJiioxus Janceolatus alone has colourless blood been observed; J. Mueller 

 Ueberder Bauu. die Lehcnscrsclicinunyen des Branchiostoma hihricum Costa, AmpMoxus 

 lanceolatus Yakrell. Berlin, 1844 {Physik. Ahhandl. der Akad. d. Wiss. zu Berlin, 

 iJahrgang, 1842). 



VOL. ir. 1 



