REPORT ON THE NEMERTEA. 81 



It is certainly all the more remarkable that iu the Hoplonemertea we nevertheless 

 find such very distinct traces of the medio-dorsal medullary nerve, notwithstanding the 

 disappearance of the plexus. And more remarkable still that this remnant — not dis- 

 tinctly traceable in only one sjjecimen of Amphiporus, whilst other specimens of the 

 same species still have it, and whilst it is even very conspicuous in Drepanophorus and 

 others — should occupy the same position as it does in the most primitive Palseonemertea, 

 i.e., in or even outside the basement membrane of the integument. This is another 

 argument for directly deriving the Hoplonemertea from the Palseonemertea. Cephalothrix 

 may be said to fill up part of the distance which separates Carinina and Carinella fi"om 

 the Hoplonemertea as far as the situation of the nerve-system is concerned (cf. PL XL 

 fig. 5), whereas Eiipolia may be said to do the same with respect to the ciliated grooves 

 on the head, and partly also to the posterior brain-lobe, its glandular investment, and the 

 long duct leading from it to the exterior. 



We must now pass on to the description of the brain-lobes and the lateral nerve-stems. 



It is known that these offer the lowest degree of specialisation in the more primitive 

 genera of Palseonemertea, e.g., Carinella. For this genus the brain- and nerve-stems 

 have been sufficiently described before (IX), and, in comparing this with what we find in 



A.T. 



PL 



Fig. 5. — Side view of the brain of Carinina iu outline, reconstructed from the sections. The fibrous core is indicated by a 

 dotted line. A.I., anterior lobe ; P.I., posterior lobe ; Co., ventral commissure ; Ln, Lateral nerve-stem. 



Carinina, the latter genus must be recognised as representing in this respect a somewhat 

 higher scale in the developmental series. This higher development finds its expression in 

 the presence of a posterior brain-lobe, comparable to the same lobe of the Schizonemertea 

 which was often designated as the side organ, although it is formed of nerve-substance 

 directly merging into that of the brain. Carinella inexpectata has been formerly 

 shown (VIII ) to possess a ciliated passage leading into the brain-substance, without any 

 special differentiation of that portion of the brain into which this ciliated channel pene- 

 trates. In Carinina such a differentiation has set in, and the brain-substance, into which 

 a ciliated canal leads, has become a separate lobe.^ In consequence of this we are, 

 moreover, enabled to draw a general — though by no means a sharply defined — distinction 

 between the portion of the brain-mass with which this accessory lobe is in contact, and 



' Chapuis has lately noticed posterior brain-lobes in a Cephalotlirix {Arch. d. Zool. Exp., vol. iv. p. xxi., 1886). His 

 description is, however, very incomplete. 



(zool. chall. EXP. — PABT Liv. — 1887.) Hhh 11 



