NERVOUS SYSTEM OF INSECTS. 493 



proach each other; the thoracic ganglia, in parti- 

 cular, coalescing into larger masses, and becoming 

 less numerous ; some being apparently obliterated : 

 the whole cord becomes in consequence shorter, and 

 the abdominal ganglia are carried forwards. The 

 optic nerves are greatly enlarged during the latter 

 stages of transformation ; and each of them is often 

 of greater magnitude than the brain itself. A set 

 of nerves also exists, the course of which is peculiar, 

 and appears to correspond with the sympathetic or 

 ganglionic system of nerves in vertebrated animals; 

 while another nerve resembles in its mode of distri- 

 bution, the pneumo-gastric nerve, or par vagum. 

 Mr. Newport has distinctly traced a separate ner- 

 vous tract, which gives origin to the motor nerves, 

 while the subjacent column sends out the nerves of 

 sensation.* 



In the next great division of the animal king- 

 dom, which includes all molluscous animals, the 

 nervous ganglia have a circular, instead of a longi- 

 tudinal arrangement. An approximation to this 

 type is manifested in the Echinodermata : for we 

 find that in the Asterias, the nervous system (Fig. 

 445) is composed of small ganglia, equal in number 



literation in the imago ; a change which those in front of them (4, 5) 

 have already undergone. The progressive developement of the 

 optic (18) and antennal (19) nerves may also be traced, Mr. New- 

 port has also traced a set of nerves (20) which arise from distinct 

 roots, and which he found to be constantly distributed to the organs 

 of respiration. 



A detailed account of the anatomy of the nervous system of the 

 Sphinx ligustri, and of the changes it undergoes during all its 

 transformations, is given by Mr. Newport in a paper in the Phil. 

 Trans, for 1832, p. 383 ; and for 1834, p. 389. See also the article 

 Insect, in Todd's Cyclopfedia of Anatomy and Physiology. 



* Phil, Trans, for 183(i, p. r,41. 



