260 



THE PROTOCHORDATA. 



In the Annelids, in contrast to the Nemertines, the gan- 

 glion-cells are not distributed uniformly along the whole 

 length of the nerve-cord, but are collected together to 

 form definite ganglionic swellings. 



It is, therefore, very significant that in the Nemertines 

 we have a median dorsal " medullary " nerve, in addition 

 to the elements which constitute the ventral nerve-cord of 

 the Annelids. 



In many Nemertines the dorsal and lateral nerve-cords 

 do not continue to lie in the ectoderm throughout life, but 



d.n 





Fig. 124. Diagrammatic view of anterior portion of a Nemertine, from the 

 left side. (After HUBRECHT, from LANG.) 



a.l. Anterior lobe of brain. /./. Posterior lobe of brain, p. Opening of pro- 

 boscis, m. Mouth, d.n. Dorsal nerve, l.n. Lateral nerve, r.n. Ring-nerves. 



sink deeper into the body, and so come to be separated 

 from the ectoderm, first by the basement membrane, and 

 then by one or more muscular layers of the body-wall. In 

 the Hoplonemertea (those in which the proboscis is armed 

 with stylets) the medullary nerve is absent. In all cases, 

 however, the longitudinal nerve-cords remain connected 

 with one another by a more or less plexiform arrangement 

 of nerve-fibres ; although sometimes a more definite con- 

 nexion, by means of metameric ring-nerves, has been 

 observed by HUBRECHT (Fig. 124). 



