Nervous System of the Chcetopoda. 313 



whole nervous system from front to back ; 2, running from 

 beliind forwards; and 3, starting in each segment on each 

 side from the nervous system and running to the periphery. 



If we trace the nervous system of Sthenelais in cross sec- 

 tions from before backwards we find even in the posterior part 

 of the brain a colossal ganglion-ceil on each side, which sends 

 its large process first of all forward for some distance into the 

 brain, and then through the oesophageal commissures into the 

 ventral cord. Here the two nerve-fibres, after a short course, 

 unite into a single one, which runs ventrally on one side of 

 the ventral cord to the posterior extremity of the body. This 

 colossal nerve-fibre is enveloped by a fibrous sheath, which is 

 at first closely applied to it, but in its further course sepa- 

 rates from it and then encloses a cavity, which constantly 

 becomes larger posteriorly and in the middle of the body 

 attains an enormous diameter. In this region also the nerve- 

 fibre, which almost disappears in its wide sheath, becomes 

 essentially modified ; it shows everywhere on its surface 

 denticulations of different sizes, which frequently pass into 

 fine processes, traversing the whole cavity and apparently 

 penetrating into the sheath. Towards the posterior extremity 

 of the body the cavity becomes smaller, until the nerve-fibre 

 again almost completely fills the sheath, and thus conditions 

 corresponding to those of the anterior extremity are re-esta- 

 blished. 



At the commencement of the ventral cord on each side 

 there are associated with this colossal nerve-fibre five others 

 of exactly the same structure. Soon after the union of the 

 oesophageal commissures with the ventral cord there are on 

 each side two colossal ganglion-cells placed ventrally, the 

 processes of which penetrate into the nervous system and pass 

 over to the other side to run backward, closely applied to the 

 median partition- wall which here divides the ventral cord into 

 two. Almost at the same time two lateral colossal ganglion- 

 cells on each side send their nervous processes transversely 

 through the ventral cord towards the opposite side, where they 

 run almost exactly in the middle to the end of the body. 

 Close behind these ganglion-cells a fifth finally occurs on each 

 side, the process of which does not pass to the opposite side, 

 but bends in the longitudinal direction immediately after its 

 entrance into the ventral cord. 



But, as already indicated, colossal nerve-fibres traverse the 

 whole ventral cord, running not only from before backwards, 

 but also in the opposite direction. 



Thus at the commencement of every body-segment, except 

 only about the anterior sixteen, there is placed laterally, but 



An7i. cfc Mag. N. Ilist, Ser. 5. Vol. xviii. 21 



