22 OLIGOCHAETA 



seen that in Chaetogaster cristallinus the connectives uniting the first, second, third 

 ventral ganglia are most distinctly double, the interspaces left being wide. The same 

 holds good with the species Chaetogaster- diaphanus ; but here the ganglia themselves 

 are distinctly separated, which is not the case with the other species ; each ganglion 

 or each half gauglion is connected with its fellow by a short commissure which give 

 to the anterior part of the nerve-cord a ladder-like appearance. 



For the remarkable specialisation of the nerve-cord in certain segments of Phreoi-yctes 

 and various Enchytraeidae cf. under the descriptions of those families. 



The ventral nerve-cord gives off branches in each segment. These branches arise 

 in two different ways ; in the earthworms and in many of the aquatic genera they 

 arise on either side of the nerve-cord and lie in the body-cavity for a greater or 

 shorter distance until they plunge into the thickness of the body-wall. In -the 

 Enchytraeidae, on the other hand, in Phreodrilus, and in many if not all Lumbriculidae, 

 the two nerves are so closely applied to each other that they appear to be only a 

 single nerve given off from the ventral side of the cord ; this apparently single nerve 

 plunges at once into the thickness of the body-wall, and then runs to right and left. 

 This peculiar state of affairs has perhaps led some observers to abstain from figuring 

 or to deny the existence of the branches of the nerve-cord; on a dissection of such 

 worms as show this origin of the branches from the ventral surface of the cord no 

 nerves would be apparent. 



In the Enchytraeidae judging from the figures of Michaelsen the unpaired character 

 of the ventral nerves is the most marked ; in RhynchelTnis, on the other hand, the 

 two nerves, although lying close together, are quite distinct as two nerves. They are 

 not present in the middle segment. In the Tubificidae and among earthworms the 

 nerves arising from the ventral nerve-cord do not at once enter the body-wall, but 

 pass to a point at some distance from their origin before they enter the body-wall ; 

 the distance varies in different species ; as a rule in those species with paired setae 

 the nerves enter the body-wall near to the ventralmost seta ; when this is further 

 away from the median ventral line the nerves have a longer course through the 

 body-cavity than when it is nearer to the median ventral line. 



The number of nerves given off in a segment varies considerably in different genera 

 of Oligochaeta. In Lumbriculus and Rhynchelmis Vejeovsky could only find a 

 single pair; three pairs is a much more usual number. This occurs for example in 

 apparently aU earthworms ; it is figured for instance by Peerier in Pontodrilus and 

 Pontoscolex. 



In these and other earthworms the three pairs are not given off at equal distances 

 from each other ; two pairs are quite close at their origin ; FriedlIndek has pointed 



