20 OLIGOCHAETA 



Pekriek for Pontodrilus; on the other hand, according to the last named author, 

 Pontoscdex has two pairs of nerves arising separately from the braia, a pair of course 

 on each side. Spencer figures only one pair of nerves in Megascoiides australis, which 

 arise, as usual, near to the circumoesophageal commissures. Thus it appears general 

 among the higher Oligochaeta for there to be only a single pair of cerebral nerves, 

 which may however, and usually do, divide at once into two. I shall point out later, in 

 describing the peripheral nerves given oflF from the ventral cord, the fact that there are 

 there three pairs in each segment; one would suppose that there would be a corre- 

 spondence between the cerebral ganglia and any one of the ventral ganglia, considering 

 that they are developed as one continuous whole. As a matter of fact this correspondence 

 exists, but it is masked by the origin of the third pair of nerves from the commissure, 

 and not from the cerebral ganglia themselves. There is nothing extraordinary in this, 

 for in the ventral ganglia one of the three pairs of nerves also arises from the commis- 

 sural part of the cord. There is thus really a correspondence between the ' cerebral ' and 

 the ' spinal ' nerves. How for does this hold good in other groups 1 The most careful 

 figures known to me of the nervous systems of the lower Oligochaeta are those of 

 Vejdovsky and of Stoi-c of the Tubificidae. In Ilyodrilus the latter figures three pairs 

 of cerebral nerves, and also three pairs in each segment arising from the ventral cord. 

 In Spirospevma there are four pairs of nerves springing from the cerebral ganglia, and 

 also four pairs from the nerve-cord in each segment ('i), one being commissural. In 

 Monopylephorus, however, there does not, it must be admitted, appear to be the least 

 correspondence. 



The cerebral ganglia are united with the ventral chains by the commissures which 

 embrace the gullet ; from the commissures arise the visceral ganglia. 



Visceral Nervous System. This appears to occur in most, if not in all, Oligochaeta. 

 In the earthworms it has been figured and described in Pontodnlus, Pontoscolex, 

 Megascolides, Hormogaster, &c. I have never found it to be wanting in any earthworm 

 where I have looked for it. It consists of either a solid mass given off' from the com- 

 missure or of a plexus having a similar origin; the plexus, however, is not entirely 

 formed of nerve-fibres ; there are also ganglionic cells ; these branches of the visceral 

 system ramify in the coats of the buccal cavity and pharynx. Among the lower 

 Oligochaeta the same visceral nerves are met with ; thus Vejdovsky figures in Chae- 

 togaster a pair of ganglia on either side of pharynx, which are connected with the brain. 

 Other groups have the homologous ganglia, and reference must be made to Vejdovsky's 

 work for further details. 



Nerves of lateral line. In many Oligochaeta — and its occurrence is probably 

 general — there is a nerve on either side of the body arising from the brain or from 



