STUDIES OX THE ECTOPARASITIC TREMA.TODES OF JAPAX. 75 



8. The Nervous System. 



The brain is situated in the anterior part of the body. In Tri- 

 stoinum, Calicotyle, Epibdella, and Monocotyle, it is in front of the 

 pharynx ; in Microcotyle, Octocotyle, Diclidopho'ra, Hexctcotijle, and Axine, 

 it is behind, while in Oncliocotyle it is just over, the same organ. It is 

 always situated on the dorsal side of the body, and is a somewhat cylin- 

 drical nervous mass crossing the median line. From it« is given off 

 both forwards and backwards a certain number of nerves. In all the 

 species that I have examined, tw-o pairs of nerves proceed posteriorly 

 from the two lateral ends of the brain, which may be distinguished as 

 the external'^ and the internal lateral nerves. The internal lateral nerves 

 are by far larger than the external, and proceed in most species along 

 the main trunks of the intestine towards the posterior end of the body. 

 In Tristoinum, it is about midway between the lateral border of the body 

 and the median line, and runs almost parallel to the former. In this 

 genus the two lateral nerves of each side unite with each other on enter- 

 iug the posterior sucker, and the single nerve thus formed immediately 

 divides into a certain number of smaller branches, which supply the 

 various parts of the sucker. Thus, a nerve is given off towards the 

 anterior part, which runs concentrically with the circumference of the 

 sucker, and after giving off at intervals some lateral branches, unites 

 at last with its fellow of the opposite side ; so that this nerve 



or many cells. Its form and function are independent of tke number of component cells . 

 Cells multiply, but the organ remains the same throughout" (" The Inadequacy of the Cell- 

 theory of Development" in Journal of Morphology, toI. VIII, 1893, p. 645); thus extending 

 the homology not only to the Xemertinei but also to the annelids. Monticelli, on the other 

 hand, in his " Primo contribute di osservazioni sui Distomidi," which I received after the above 

 had been written down, homologises the terminal funnel of Plathelminthes with the head- 

 kidney of amelids (cf. L c. p. 45 et infra). Schuberg is reported to have arrived at a similar 

 conclusion concerning the nature of the wall of the excretory capillaries. 



1). In my paper on Diplozoon I have called this pair the ventro-lateral nerves, but the 

 terms above mentioned are better as being of wider application. 



