NERVOUS ACTIONS IN ANNULOSA. 333 



government of the remarkable locomotive powers of these animals is thus 

 provided for, their active respiratory functions and other nutritive processes 

 assisting and giving them the requisite muscular irritability, whch is so strik- 

 ing when contrasted with the slow movements of the Mollusca. 



Instead of the parieto-splanchnic ganglia of the Mollusca, there is found, 

 at least in the higher Annulosa, a very remarkably complete sympathetic 

 system. Even from the cephalic ganglia two minute filaments are given 

 off, which speedily unite to form a single cord, on which a minute ganglion is 

 found ; and from this, branches proceed to the alimentary canal, the dorsal 

 vessel, and the adjacent large tracheae ; from the commissural bands or tracts 

 between the several ganglia, similar nervous filaments arise, which unite, are 

 connected with a minute ganglion, and give off branches chiefly for the dorsal 

 vessel and tracheae of particular segments. These minute ganglia and nerves 

 must govern, like the sympathetic system of the Yertebrata, the vegetative 

 processes of the animal, viz., those performed by the alimentary canal, the 

 glands, the dorsal vessel, and the tracheae. 



The nervous system of the Annulosa, is modified, so as to be adapted to the 

 varieties of form in the heads and bodies of the different classes or groups. 

 Thus, the development of the cephalic ganglia, corresponds exactly with that 

 of the parts situated on the head, and of the organs of the senses, especially of 

 the eyes. Again, the number, size and degree of concentration, of the series 

 of abdominal ganglia, correspond with the number of the segments in the body, 

 their size, the degree of development of their attached limbs, and the mode in 

 which two or three segments are sometimes fused together. In the tailed 

 Crustacea, as in the lobsters, shrimps, and others, the "thoracic segments are 

 consolidated, and the thoracic ganglia are concentrated into a single large 

 mass, placed at a considerable distance from the cephalic ganglion, the con- 

 necting commissures between which are therefore unusually long ; this thoracic 

 ganglion supplies the nerves to the claws and feet ; the abdominal ganglia, in 

 accordance with the length and subdivisions of the trunk, are numerous and 

 separate. On the other hand, in the tailless Crustacea, as in the crab tribe, 

 in which the body is mainly composed of the wide and consolidated thoracic 

 segments, and the abdomen is, as it were, atrophied, the nervous system con- 

 sists of a cephalic ganglion, placed in the head, and connected, by the usual 

 commissural nerve cords, with a single thoracic ganglionic mass, in which, 

 all the locomotive ganglia are concentrated, and from which the nerves radiate 

 to the several feet ; in this most perfect example of the concentration of the 

 inferior ganglia, the resulting nervous mass may be aptly compared to a Mol- 

 luscous pedal ganglion. In the Arachnida, or spiders, the inferior portion of 

 the nervous system, is also remarkably concentrated, there being usually a 

 large thoracic ganglion, and a large single abdominal ganglion. Finally, in 

 the Myriapoda or Centipedes, the nervous system, in accordance with the 

 repetitive segmentation of the body, the number and equality of its component 

 segments, the absence of any specially developed locomotive members from 

 any one segment, and the presence of numerous members of nearly equal de- 

 velopment upon all the segments of the body, presents a great number of ab- 

 dominal ganglia, a pair being found in each segment of the trunk, differing, 

 but little in size, one from the other. 



Most remarkable instances of the pliability of organic types, may be seen in 

 the singular modifications of the nervous system of the same individual insect, 

 during the metamorphosis which the true insects undergo, from the larva or 

 caterpillar state, to the pupa or chrysalis, and then to the imago state. In 

 each of these changes, the nervous system participates. Thus, in the cater- 

 pillar, or grub, in which the segments of the body are always more numerous 

 than in the perfect insect, the number of the abdominal ganglia is increased 

 accordingly ; and, as these several segments in the caterpillar, are of nearly 

 equal importance, and equally developed, the ganglia are of nearly equal or 

 uniform size. Moreover, as the organs of special sense, of which the eyes are 

 still the most important, are comparatively feebly developed, the cephalic gan- 

 glia or special sensorial centres, are relatively small, and present no marked 

 preponderance over the abdominal ganglia. The locomotive powers of the 

 caterpillar, or grub, are also singularly limited, in comparison with those of 



