STUDIES ON THE ECTOPARASITIC TKEM.VTODES OF JAPAN. 75 



8. Tlic Nercons SijMciii. 



The hmin is situited in the anterior part of the body. In Tri- 

 slomniii^ CaUcotijle, Epthdella, and Monocolijlc, it is in front <jf tlie 

 pharynx ; in Micvocoiijlc^ Odocolijlc^ Dididophoiu, Hexacohjle, and Axinr, 

 it is behind, while in Oncliocoti/Ic it is just over, the same organ. It is 

 always situated on the dijrsal side of the body, and is a somewhat cyKn- 

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

 l)oth forwards and Ijackwards a certain number of nerves. In all the 

 s[)ecies that I have examined, two pairs of nerves proceed posteriorly 

 from the two lateral ends of the brain, wliich may be distinguislicd as 

 the cxtcrnaV and the {ntcrnal lah'ral Jiervcs. The internal lateral nerves 

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

 the main trunks of the intestine towards the posterior end of the l)0(lv. 

 In Trisloimun^ it is about midway between the lateral Ixn-der of the body 

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

 o'enus the two lateral nerves of each side unite with each other on enter- 



o 



iiig 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 towiU'ds the 

 anterior part, which runs concentrically with the circumference of the 

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

 at last with its fellow of the o[)posite side ; so that this nerve 



or niauy cells. Its form and functiou are independent 01 the number of component colls . 

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

 theory of Development" in Journal of Morphology, vol. VIII, 1893, p. 615); thus extending 

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

 hand, in his " Primo contributo 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. I. 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. 



