THE ANATOMY. VISCERAL NERVOUS SYSTEM 21 



the oesophageal commissure, which has been compared to the nerve of the lateral 

 line in fishes; it appears to consist especially of nerve-cells. This system of nerves 

 has been described by Vejdovsky as existing in the Enchytraeidae, Phreoryctidae, 

 Naidomorpha, Tubificidae, and Lumbriculidae^- Connected with it is a system of fine 

 fibres which supply the walls of the alimentary canal ; in Chaetogaster the oesophagus 

 has a ling of nerve-cells round it, which are apparently referable to this system of 

 intestinal nerves, which may fairly be compared with the Sympathetic system of 

 Vertebrates. This nervous supply of the alimentary canal will not of course be 

 confounded with the visceral nerves ah-eady described as arising from the circumoe- 

 sophageal commissure. The lateral nerve itself or rather ganglionic chain originates 

 from the epidermis, and remains in connexion with the same, the longitudinal muscles 

 being separated where it occurs. 



Ventral Nerve-Chain. The commissures which arise from the brain and embrace 

 the gut unite below it to form a ganglionated chain. This runs from end to end of the 

 body ; in the extreme posterior region, where a regeneration of segments is going on, 

 the ventral nerve-cord may be often seen to lie in the thickness of the epidermis ; 

 otherwise it always lies in the body-cavity ; but Vejdovsky states that it is for the most 

 part naked ; that is, not covered by a continuous coating of peritoneum ; scattered 

 cells of the peritoneum are attached to it here and there. The nerve cord is usually 

 enclosed in a muscular sheath, which may be, or is sometimes not, continuous right 

 round it; but this nerve-sheath is derived from the same embryonic cells as those 

 which form the cord itself. The cord may be divided into the ganglionic and the non- 

 ganglionic or ' connective ' part ; the degree to which these are diSerentiated varies ; in 

 Chaetogaster for instance, they are sharply marked off from each other in the figures given 

 by Vejdovsky ; on the other hand it is the rule among earthworms for there to be only 

 a slightly marked distinction ; Spencer even went so far as to practically deny, in 

 the case of Megascolides, any difference in the whole length of the cord; but Vej- 

 dovsky found constrictions separating the pairs of ganglia. In any case it seems certain 

 that the ganglionic part of the cord fades gradually into the connective region, and 

 the latter when present is of so short an extent that it is hardly recognisable. It is 

 remarkable that the higher Oligochaeta should in this respect also show more primitive 

 characters than the lower forms. 



The primitively double character of the ventral nerve-cord is partly retained for 

 life in the genus Chaetogaster. In the Plates illustrating Vejdovsky's work upon 

 the Oligochaeta there are several figures of the nerve-cord of this genus ; it will be 



' Hesse however states that the supposed ganglion cells are only the non-modified protoplasmic portion 

 of the ' nematoid ' muscular fibres in Enchytraeidae and Naidomorpha. 



