HOMOGANGLIATA. 187 



formed, passing along the whole length of the body. With the 

 exception of the anterior pair of ganglia, or that contained in the 

 first ring, which we may call the head of the worm, the nervous 

 centres are arranged along the ventral region of the body, that is, 

 beneath the alimentary canal ; but the anterior pair itself is inva- 

 riably placed upon the dorsal aspect of the animal, and communi- 

 cates with the rest by a nervous collar which embraces the com- 

 mencement of the oesophagus. The nervous masses placed along 

 the belly would seem to preside specially over the movements of 

 the segments to which they belong, and to have little to do with 

 sensation or the perception of external objects ; whilst the anterior 

 or cephalic pair, from the constancy of their communication with 

 the organs of the senses, would appear peculiarly in relation with 

 the perceptive faculties of the creature. 



(230.) It may be taken as a general law, that the perfection of 

 the nervous system of any animal may be estimated by the propor- 

 tionate size of the central ganglia connected with it, upon the 

 developement of which both the energy of the actions of the 

 body and the completeness of perception depend ; and, by follow- 

 ing out this great principle, we shall be easily able to account for 

 the progressive steps by which the Articulata become more and 

 more perfectly organized, as we trace them in the series above in- 

 dicated. In proportion as we have found the segments of the 

 body to become less numerous, the appended limbs stronger, the 

 outward skeleton more dense, and the muscular powers more ener- 

 getic, we shall find the abdominal ganglia to dimmish in number 

 by becoming consolidated into larger masses, increasing in size and 

 energy in accordance with the developement of the limbs over 

 which they preside : and in the same manner we shall observe the 

 senses assume greater perfection of structure, and the instincts 

 become more developed, as we find the cephalic or anterior pair of 

 brains increasing in proportionate bulk. 



These observations will suffice to introduce the student to the 

 Homogangliate division of the animal world, and to direct his at- 

 tention to those physiological points connected with the nature of 

 their nervous system which will be more fully laid before him in 

 the following pages. 



