>LOGY FOR MEDICAL STUDENTS CHAP. 



central part of the nervous system becomes more and more sharply 



a peripheral portion which, composed mainly of nerve- 



: vly to convey the nerve impulses to or from the nerve 



In this respect the nervous system of the earthworm shows 



marked advance on that of a Coelenterate ; inasmuch as the 



portion of the nervous system is sharply marked off and com- 



highly developed. It consists firstly of a longitudinal 



ventral nerve-cord which runs throughout the length of 



rm in the mid-ventral line and immediately internal to the body- 



.-.'.<-. and Fig. 63, A T ). In each somite the cord is slightly 



>U( ,llen these swellings or ganglia being simply portions of the cord 



in which there is a specially marked aggregation of ganglion-cells. From 



lion there pass off to each side slender nerves, i.e. bundles of 



fibres, some of which are motor, connected with the muscles of 



the bodv-wall. while others are sensory, ending in sensory cells in the 



rmis. 



Secondly, in addition to the ganglia of the ventral cord there are 



present a pair of ganglia (cerebral, or supra-oesophageal ganglia 



v.0.), which lie side by side, dorsal to the pharynx and close 



front end. These are continuous with one another through a 



commissure or bridge of nerve-fibres, while each is also continued 



outer side into a eircum-oesophageal commissure which curves 



round the side of the alimentary canal and is continued ventrally into 



the first ganglion of the ventral cord. From the cerebral ganglion on 



here passes forwards a conspicuous little nerve consisting 



mainly of sensory fibres connected with sensory cells in the epidermis 



of the prestomium, this latter being an extremely sensitive organ by 



\vhi< h the worm, so to speak, feels its way when burrowing through 



the earth. 



if the Earthworm serves to illustrate a number of important 



i prim iples of animal structure. The Coelenterates and Sponges 



Cfl either sessile in habit (i.e. fixed in one spot) or capable 



aparauvely sluggish and indeterminate movements. The 



thcr hand moves about actively and its movements are 



in relation to the structure of its body one particular end 



front under normal circumstances, and one particular 



side being above. Correlated with this type of movement, the body of 



undergone adaptive evolution in its general structure. 



It has become elongated in the line of movement. Its two ends have 



iated though not so markedly as in many other worms 



