NATURE OF THE IN VERTEBRATE BRAIN. 705 



a single lower ganglion ; and, as a consequence of the coalescence of 

 the two lower halves, we have, instead of the two lateral cords of 

 the Nemertidce, a double ventral nervous cord traversing the whole 

 length of the body. There are no distinct ocelli in the earthworm. 

 The body is composed of a multitude of ring-like segments, each of 

 which is provided with lateral seta3, which are called into play during 

 the subterranean locomotions of the animal. 



The double ventral cord has a fibrous structure along its upper 

 surface, while below there is an irregular stratum of ganglion-cells. 

 These cells are more abundant about the centre of each body-seg- 

 ment, and their aggregation gives rise to a series of rudimentary 

 ganglia in these situations. From, every one of the ganglionic swell- 

 ings two nerves are given off on each side, while a third pair of 

 nerves issues from the cord itself just anterior to the swelling, and is 

 distributed along the anterior boundaries of the segment. 



The oesophageal ganglia in the earthworm are, proportionately to 

 the rest of the nervous system, much smaller than in the Nemertidce ; 

 and this is perhaps due in great part to the existence of the numer- 

 ous segmental ganglia in the former, which have no existence in the 

 marine worms. The movements of the Nemertidce, like those of the 

 nematoids, are probably much more exclusively under the control of 

 the oesophageal ganglia than are those of the segmented earthworm 

 in which each of the body-ganglia doubtless has much to do with 

 bringing: about the contraction of contiguous muscles. The earth- 

 worm has also a more complex visceral structure than is to be met 

 with among the Nemertidce; and, moreover, it presents more dis- 

 tinct evidences of a nervous interconnection between the different 

 organs of the body and some of the principal nerve-centres. Lock- 

 hart Clarke has described a complicated ganglionic network on each 

 side of the oesophagus, starting from the commissures and sending 

 prolongations to the intestine and other parts. By means of this 

 principal visceral system of nerves, the internal organs are brought 

 into relation with one another, and with the nervous system of ani- 

 mal life that is, with those parts having to do more especially with 

 the relation of the organism to its medium. 



The upper or supra-cesophageal ganglia, representing the brain of 

 the earthworm, receive a nerve-trunk on each side, composed of fibres 

 coming from the tactile upper lip, and, as no sensory filaments of a 

 different order are known to be immediately connected therewith, the 

 functions of the brain in this animal must be comparatively simple. 

 This upper lip contains a certain amount of diffused pigment, though 

 there are no signs of the existence of distinct ocelli. I have spoken 

 of the part as a special organ of touch, but it is equally probable that 

 it may be capable of receiving more special impressions representing 

 rudimentary tastes. The separation between these modes of sensi- 

 bility may in such low organisms be somewhat indefinite. 

 vol. ix. 45 



