142 ZOOLOGY FOR MEDICAL STUDENTS chap. 



that a central part of the nervous system becomes more and more sharply 

 marked off from a peripheral portion whichj composed mainly of nerve- 

 fibres, serves merely to convey the nerve impulses to or from the nerve 

 centres. In this respect the nervous system of the earthworm shows 

 a very marked advance on that of a Coelenterate, inasmuch as the 

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

 paratively highly developed. It consists firstly of a longitudinal 

 strand — ^the ventral nerve-cord — which runs throughout the length of 

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

 wall (Fig. 65, B, n.c, and Fig. 63, N). In each somite the cord is slightly 

 swollen — these swellings or ganglia being simply portions of the cord 

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

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

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

 the body-wall, while others are sensory, ending in sensory cells in the 

 epidermis. 



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

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

 Fig. 65, B, s.o.g), which lie side by side, dorsal to the pharynx and close 

 to its front end. These are continuous with one another through a 

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

 at its outer side into a circum-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 

 each side there 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 

 which the worm, so to speak, feels its way when burrowing through 

 the earth. 



The study of the Earthworm serves to illustrate a number of important 

 general principles of animal structure. The Coelenterates and Sponges 

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

 of only comparatively sluggish and indeterminate movements. The 

 worm on the other hand moves about actively and its movements are 

 determined in relation to the structure of its body — one particular end 

 always going in front under normal circumstances, and one particular 

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

 the creature has undergone adaptive evolution in its general structure. 

 It has become elongated in the line of movement. Its two ends ha■<^e 

 become differentiated — though not so markedly as in many other worms 



