HOMOGANGLIATA. 215 



ARACHNIDANS, or Spiders, still further consolidation of the external 

 skeleton is visible ; for, in them, even the separation between the head 

 and the thorax is obliterated, and it is in the abdomen only that the 

 segments of the body are recognizable. 



(561.) Lastly, among the CRUSTACEANS we have various modifications 

 of the outward skeleton, adapted to the habits of the different tribes. 

 In the least perfect species, which are all aquatic, the segments 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 con- 

 struction of the calcareous shield that covers and protects their whole 

 body. 



(562.) We see, therefore, from the above rapid sketch of the different 

 classes composing 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 more immediately connected with 

 locomotion or the destruction of prey ; let us next examine the nature 

 of the nervous apparatus that characterizes the HOMOGANGLIATA, and 

 observe the relation which the outward form of the body bears to the 

 arrangement of this primary system of the animal economy. In the 

 humblest forms of the Annulosa, it would seem that every ring of the 

 body contains 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 formed, passing along the whole length of 

 the body. With the exception of the anterior pair of ganglia, or those 

 contained in the first ring, which we may call the head, the nervous 

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

 beneath the alimentary canal ; but the anterior pair are invariably 

 situated upon the dorsal aspect of the animal, and communicate with 

 the rest by a nervous collar that embraces the commencement of the 

 oesophagus. The nervous masses placed along the belly preside specially 

 over the movements of the segments to which they belong, and have 

 little to do with sensation, or the perception of external objects ; whilst 

 the anterior or cephalic pair, from the constancy of their communi- 

 cation with the organs of the senses, appear to be peculiarly in relation 

 with the perceptive faculties of the creature. 



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

 nervous system of any animal may be estimated by the proportionate 

 size of the central ganglia, upon the development of which both the 

 energy of the actions of the body and the completeness of perception 



