188 AN INTRODUCTION TO ZOOLOGY 



sensations of touch, taste, smell and light, but so far as we can 

 ascertain it is incapable of receiving sound impressions. The sense 

 of touch is the most widely distributed of them, and is spread all over 

 the body. Next to that is the sense of light, which is present all over 

 the anterior end but is absent from the posterior half of the worm. 

 The senses of taste and smell are more limited in their extent, being 

 confined almost entirely to the prostomium, the peristomium near 

 the mouth, buccal cavity and pharynx. 



Let us now consider the. mode of action of the nervous system 

 as a whole. Four elements are involved in the reflex arc in the case 

 of the worm, three of them are nervous and the other a muscle. 

 The sensory receptor projecting from the epidermis with its afferent 

 process arborising in the cord is in connection with the dendron of an 

 association neuron, and the axon of this cell is in turn related to the 

 dendron of a motor cell. Thus the stimulus is transferred from the 

 skin to a motor neuron, and thence sent down, via its axon, to the 

 muscle connected with the epidermis, bringing about a movement of 

 the latter. This then is a very simple reflex arc, and although quite 

 a small stimulus is bound to affect a large number of sensory cells 

 and an even larger number of motor neurons the process is essentially 

 the same. The cerebral ganglia certainly play a part in co-ordinating 

 the movements of the animal as a whole, but not to nearly such a 

 large extent as does the brain of the frog, and the main activities of 

 the worm are the result of these reflex actions. This is most clearly 

 seen when a worm is cut to pieces, for each part, although entirely 

 removed from any connection with the cerebral ganglia, still ex- 

 hibit normal movements, which must of necessity be reflexes, and the 

 ganglion pair in each somite is the centre of such actions for its own 

 segment. The presence of the commissural neurons with their well- 

 branched axon and dendron makes it possible for a sensory impulse, 

 even coming from a limited area, if sufficiently strong, to be dis- 

 tributed to a fairly large number of motor cells, and consequently 

 to bring about adjusting movements on the part of larger portions of 

 the body or even of the animal as a whole. By these means, then, 

 the movements of the worm in response to the stimuli produced by 

 changes in its environment are brought about. 



An examination of a transverse section of Lumbricus 

 when compared with one of Hydra will reveal certain of the impor- 

 tant differences, not merely between the animals themselves, but 

 between the Ccelomata and Coelenterata. In Hydra, as we have seen, 

 the body wall is simple, consisting of an ectoderm and an entoderm 

 with the intermediate mesoglea, and it bounds a single internal 

 cavity, the coelenteron. In Lumbricus the body wall is composed of 

 five distinct layers, i. The cuticle is the outermost, and consists 



