Mr. J. L. Clarke on Lumbricus terrestris. 255 



municating in part with the other in the mesial line, where they form 

 a kind of indistinct decussation in front of the notch. 



Distribution of the Cephalic Nerves. — Their roots on each side 

 immediately separate into two trunks, a lower and upper. The 

 former runs above the mouth, to the under side of the first conical 

 segment, or upper lip. Here it divides into several branches, which 

 supply its muscular bands, and then terminate in the integument as 

 a plexus, which appears to communicate with that from the first 

 enlargement of the pharyngeal chain, spread over the tubular mouth, 

 which is itself continuous with the upper lip. The upper trunk 

 proceeds directly to the corresponding part of the same segment, and 

 there divides into two branches, of which one in particular, after 

 running the course of the pigmentary laminae, and giving oif a series 

 of short filaments, terminates at the point, beneath the integument. 



In the pigmentary laminse the nerves form an intricate plexus, 

 and the impression was that many of their ultimate fibres end in 

 loops. They were never seen to be directly connected with the large 

 clear cells scattered through the substance. Nor is there any ground 

 for conjecture with regard to the office of these cells : perhaps they 

 are intended for the transmission of light. From the structure of 

 the segment and the distribution of its nerves, it is not unreasonable 

 to think that its upper surface may be instrumental in the perception 

 of diffused light ; and that its under surface and point may be sub- 

 servient not only to the sense of touch, but perhaps also to that of 

 smell, in a low degree. That it forms an important organ of search, 

 is pretty evident from the manner in which it is projected alter- 

 nately forwards and from side to side, as the animal advances in its 

 course. 



The upper side of the cephalic ganglion corresponds to the under 

 side of the subventral, and several points of resemblance are indi- 

 cated between* the two kinds of centres. Each pharyngeal crus is 

 shown to be a compound structure, composed of different sets of con- 

 necting fibres, — 1, between its own nerves, which supply, on the one 

 side, the cephalic portion of the outer tube, and on the other, the 

 corresponding part of the alimentary tube reflected inwards from the 

 former ; 2, between these nerves and their fellows of the opposite 

 crus, across the front of the first ganglion ; 3, between the same 

 nerves and the cephalic ganglion; and 4, between the cephalic 

 ganglion and the same side of the whole subventral chain. Now 

 there is this point of difference between the two kinds of centres 

 compared together, — that while the last-mentioned set of fibres on 

 the one side is continuous with that on the other, as a transverse 

 band through the cephalic ganglion, the subventral cords, although 

 continuous with these on their respective sides, form no such con- 

 nexion with each other across the lateral halves of their own ganglia, 

 but run parallel and directly backwards through them. Such a 

 communication, however, is established for the latter, individually, 

 by the transverse fibres of their own nerves ; and just as these fibres 

 unite the lateral halves of each separate ganglion, independently of 

 the cephalic, so do the last-mentioned set of fibres of the crura con- 



