NERVOUS SYSTEM. LATERAL SYMMETRY. *79 



manifest correspondence to those of certain parts of the brain in 

 Vertebrata. They are not the representatives of the Cerebral 

 hemispheres, which are peculiar to the latter ; but they ob- 

 viously resemble the Optic and other ganglia of special sense, 

 which we find even in the highest Vertebrated animals, at the 

 origin of the nerves that proceed to those organs, and which in 

 Fishes make up so large a proportion of the entire brain, as 

 frequently to surpass in size the cerebral hemispheres themselves 

 ( 550). The ganglia of the ventral cord are so many centres of 

 reflex action to the different segments which they supply (ANIM. 

 PIIYSIOL. 442) ; and in this respect they are analogous to the 

 Spinal Cord of Vertebrated animals, from which they differ in no 

 important particular, except in their position. Even in this, 

 however, there is a greater resemblance than is at first apparent ; 

 for there is strong reason for regarding the usual position of 

 Articulated animals as really inverted ; that is, for considering 

 what is apparently the ventral surface of the body as in reality 

 the dorsal, and the apparent back as the real abdomen. 



653. The bulk of the body in the Articulata is made up of 

 the muscles, by which the several segments, and their various 

 appendages, are put in motion. These muscles, like the parts of 

 the body themselves, are arranged with great regularity and 

 exactness on the two sides of the median or central line ; so that 

 the lateral symmetry ( 47) of the Articulata is most exact. 

 We shall hereafter see that this symmetry extends in great 

 degree also to the organs of nutrition ; which in the Yertebrata 

 are not arranged with the same equality. Where the segments 

 and their appendages have a similar form and action, their 

 muscles are but repetitions of each other ; but where, as in the 

 higher Articulata, the segments and their appendages are differ- 

 ently constructed, the muscles also are more variously arranged, 

 and often form a system of great complexity. 



654. The development of the organs of Nutrition in Articulated 

 animals would seem to be altogether subservient to that of the 

 Locomotive apparatus ; their function being chiefly to supply 

 the nerves and muscles with the aliment necessary to sustain 

 their vigour. The power of these muscles is so great in proper- 



