THE ANATOMY. VENTRAL NERVE CORD 23 



out, and I confirm him (for Perichaeta), that in Lumbricus at any rate the two pairs of 

 nerves which arise close together have a relation to the ventral nerve-cord, similar to 

 that which the dorsal and ventral roots of the spinal nerves have to the spinal cord 

 of most vetebrates ; the one in fact is situated more dorsally than the other. This state 

 of aiFairs is remarkably distinct in certain species of Perichaeta which I have examined 

 from this point of view. In Pontodrilus — in the segment which contains the spermi- 

 ducal glands — there is a ganglion on one of these nerves, and just at the ganglion a 

 branch arises which goes to the other nerve ; this recalls the ganglion on the dorsal 

 root of the spinal nerves of Vertebrates and the branch which immediately after 

 unites this branch with the ventral root. Although there is this resemblance between 

 all earthworms in the number and position of the branches of the ventral nerve cord 

 in all earthworms, there is by no means a close correspondence between the various 

 genera of aquatic Oligochaeta. Moreover accounts are apt to differ in many cases. 

 For example, in • Tubifex Vejdovsky figures no less than five branches of the cord 

 in each segment ; d'Udekem gives three as the number, while Nasse only found two. 



Stolc figures five branches in Monopylephorus and Lophochaeta, two being dissepi- 

 mental branches. I found three in Phreodrilus. 



There is not much information as to the course of the branches after they have 

 left the nerve-cord; Pereiee carefully dissected out these branches in Pontoscolex ; 

 he found that one only of the three branched considerably in the thickness of 

 the body-wall. I found in the Perichaetid genus Biporoehaeta a considerable 

 branching of these trunks within the thickness of the body of the wall, resulting in 

 fact in the formation of a nerve-plexus ; on the other hand, I found in the same worm, 

 and I have noticed similar appearances in other worms, that the branches arising 

 from the cord were continuous • right round the body, apparently joining dorsally. 



The histology of the nervous system is a large subject and one which can only 

 be treated very briefly in the present work. It has been investigated by a large 

 number of obsei-vers, including Vejdovsky, Retzius, Feiedlandee, etc. In transverse 

 sections of the nerve-cord of Lumbricus, three dorsal tubes are very obvious ; these 

 'have received various names, and very various functions and homologies have been 

 assigned to them. There is now no longer any question that these tubes, the 

 ' Neurochord,' are of nervous nature ; for they have been traced into connexion with 

 nerve-cells ; there are generally three of them ; but occasionally four are present the 

 tubes dividing and reuniting. These tubes in Rhynchelmis are developed out of 

 a row of large cells which were formerly (and erroneously) regarded as being of 

 mesodermal origin. The neurochord of the adult is single in the anterior segments 

 and in the brain and the oesophageal commissures. In the middle and hinder part 



