MOLLUSCOUS, ARTICULATED, AND VERTEBHATED TYPES. 73 



50. Thus wo have seen, that there is a kind of antagonism 

 between the characters of the Molluscous and of the Articulated 

 series : for the former present the functions of nutrition in almost 

 exclusive operation, their whole energy being (as it were) con- 

 centrated upon these, and their powers of sensation and locomo- 

 tion being exercised almost exclusively for the acquirement of 

 their supplies of food; whilst in the latter, the functions of animal 

 life, sensation and spontaneous motion, are seen in the greatest 

 activity, and the nutritive operations are carried on merely for 

 the purpose of developing the mechanism in the first instance, 

 and of subsequently keeping it in repair and supplying its waste. 

 We observe a corresponding difference in the form of their bodies; 

 for the symmetrical arrangement, which almost invariably pre- 

 sents itself in the organs of animal life, is so complete in the 

 Articulata, as even to prevail in their organs of nutrition ; whilst 

 an absence of all symmetry shows itself in those Mollusks, whose 

 bodies are made up almost exclusively of these last parts ; uni- 

 formity on the two sides being only visible in those, which have 

 some capacity for movement ; and merely affecting the head in 

 those, whose motions do not exhibit more than the sluggishness* 

 which is characteristic of the series taken as a whole. 



51. Now, in the Vertebrated subdivision of the Animal 

 kingdom, we find the characters of these two groups in some 

 degree united. For there is in almost every member of it, a 

 large share of the activity and acuteness of sensation, which is 

 characteristic of the Articulated classes ; whilst the nutritive 

 apparatus is rather formed upon the plan of that of the Mollusca, 

 and attains a still higher degree of development. In the external 

 form of the body of the Vertebrata, we have, in almost every 

 instance (a small group of Fishes constituting the only excep- 

 tion), a complete bi-lateral symmetry ; and this symmetry 

 extends to the internal parts, so far as the organs of animal life 

 (the brain and nerves, the muscles concerned in the movement 

 of the body, and the organs of sense), are concerned ; but we do 

 not find it so well marked in the organs of nutrition, which are, 

 for the most part, unsymmetrical. Thus the heart of Man is not 

 placed on the central line of the body, but on the left side ; the 



