GENERAL SUBDIVISIONS OF THE ANIMAL KINGDOM. 41 



the lower classes of Plants, absorbs nourishment by its expanded surface 

 only. 



4. Some Physiologists have asserted that the nature of the respiratory pro- 

 cess affords a ground of distinction between Animals and Plants ; oxygen 

 being absorbed, and carbonic acid evolved, by the former, and a converse 

 change being effected in the surrounding air by the latter. It is not correct, 

 however, to designate this converse change as a consequence of the respiratory 

 process ; for in Plants, as in Animals, there is a continual absorption of oxygen 

 and evolution of carbonic acid, which constitute the true function of respira- 

 tion; but the effects of this change are masked (as it were), in Plants, by 

 those of the fixation of carbon from the atmosphere, which only takes place 

 under the influence of sun-light, and which is much more analogous to the 

 digestion of Animals. The most valid distinction, in doubtful cases, seems 

 likely to be founded on the chemical constitution of the tissues themselves. 

 In the plant, the whole of the organized structure, when freed from the pro- 

 ducts of secretion which are deposited in it, (many of these containing the 

 same proportion of nitrogen as exists in animal flesh,) is found to have the 

 same composition with starch ; being formed of oxygen, hydrogen, and car- 

 bon only. In the animal, on the other hand, the organised tissues all contain 

 azote as part of their proper substance ; non-azotised compounds, such as 

 fatty matter, being merely deposited in these, as products of secretion. Hence 

 if the chemical composition of the organised tissues themselves can be cor- 

 rectly determined, the Vegetable or Animal nature of a doubtful body may be 

 ascertained. By this test, the long-disputed question of the nature of the 

 true Corallines has been set at rest ; their tissue, when freed from the lime de- 

 posited in it, being found to have the composition of that of Plants : and upon 

 evidence of the same kind, (the presence of starch in their interior,) a large 

 number of tribes, which have been described by Ehrenberg as Animalcules, 

 are now generally referred to the Vegetable kingdom. 



2. General Subdivisions of the Minimal Kingdom. 



5. The animal kingdom was formerly divided into two primary groups, the 

 Vertebrated and the Invert ebrat ed; the former comprising those which are dis- 

 tinguished by the possession of a jointed spinal column, consisting of a num- 

 ber of internal bones, termed vertebrae ; and the latter including all those ani- 

 mals which are destitute of this support. It was pointed out by Cuvier, 

 however, that among the Invertebrata there are three types of organization, 

 as distinct from each other as any of them are from the Vertebrata ; and he 

 accordingly distributed the whole under four primary, divisions or sub-king- 

 doms : of these, the VERTEBRATA rank highest; next, the ARTICULATA and 

 the MOLLUSCA, which may be said to form two parallel series, both of them 

 inferior in degree of organization to the Vertebrata, but superior to the lowest 

 group ; and lastly, the RADIATA, which include those animals that border most 

 closely, both in external aspect, and in general character, upon the Vegetable 

 kingdom. The members of these groups are readily separated from each 

 other by the structure of their skeletons, or organs of support and protection ; 

 as well as by many other characters. In the Vertebrata, the skeleton consists 

 of a number of internal jointed bones, which are clothed by the muscles that 

 are attached to them and move them ; these bones are traversed by blood- 

 vessels, a.nd are to be regarded as in all respects analogous to the other living 

 tissues of the body. In the Articulata, the soft parts are supported by a hard 

 external envelope, which is of corresponding form on the two sides of the 

 median line, and which is divided into several pieces, jointed or articulated 



together by a membrane, in such a manner as still to allow of free motion ; 



4* 



