186 HOMOGANGLIATA. 



in the least perfect species, which are all aquatic, the rings of the 

 skeleton are perfectly distinct and separate, resembling those of 

 the Myriapoda ; but in the stronger and more predacious tribes, the 

 pieces of the head and thorax become solidly fixed together ; and in 

 those forms most adapted to a terrestrial life, namely, the crabs, 

 almost all traces of distinction between the thoracic segments are 

 lost in the construction of the calcareous shield which covers and 

 protects their whole body. 



(229.) We see therefore in the above rapid sketch of the dif- 

 ferent classes which compose the articulated division of the animal 

 kingdom, that, as their organization assumes greater perfection, 

 the different segments of the external skeleton coalesce and become 

 united together, so as to give greater strength to those parts which 

 are more immediately connected with locomotion or the destruc- 

 tion of prey ; let us now examine the nature of the nervous appa- 

 ratus which characterises the HOMOGANGLIATA, and observe the 

 relation which the outward form of the body bears to the arrange- 

 ment of this primary system of the animal economy. In tracing 

 the developement of animal structure, on the first appearance of 

 any new apparatus, it is by no means unusual to find it repeated 

 again and again in the same creature, divided as it were into 

 distinct portions, prior to its appearance in its more highly organ- 

 ized and perfect condition. Thus in Ccenurus cerebralis, 110, 

 the reader will remember numerous mouths were dispersed over 

 different parts of the simple sac composing the stomach of the 

 animal ; in the compound Polyps, 36, innumerable digestive 

 organs ministered to the support of one common mass ; in the 

 Tape-worm, 117, the generative apparatus was repeated in nearly 

 every segment of its compound body ; and, did we choose to antici- 

 pate, other examples might be adduced, derived from the more per- 

 fect animals, exemplifying the same fact. We shall not be sur- 

 prised, therefore, to find that, on the first developement of a 

 nervous system provided with ganglionic masses, these nervous 

 centres, or brains as we might term them, are very numerous, and, 

 instead of being united, are located in different parts of the system. 

 In the humblest forms of the Annulosa it would seem indeed that 

 every ring of the body contained a complete nervous apparatus, 

 consisting of a pair of ganglia and a set of nerves destined to 

 supply the particular segment in which they are lodged. All these 

 different brains, belonging to the individual segments, communicate 

 with each other by nervous filaments, so that a continuous chain is 



