1 86 



THE NEMERTINI 



vexity backwards, and the dorsal and ventral horns nearly meeting 

 their fellows of the other side (XXV. C) ; the bottom of this furrow 

 is subdivided into pits by tranverse ridges; the "lateral canal" 

 opens at the hindmost part of the curve. 



But in the Heteronemertini (Fig. XXV. (7, D, E) this vertical 



Fio. XXV. 



Heads of Nemertines (after Burger). A, ventral, and B, lateral views of Cannella ; C. ventral 

 view of Drepanophorus ; D, ventral view of VaLencinia ; E, lateral view of Cerebratulus ; a, 

 mouth ; b, rhynchoatome ; c, groove in side of head, vertical or horizontal, in connection with 

 the cerebral organ. 



furrow is replaced by a horizontal " cleft " deeper or shallower, 

 longer or shorter especially well developed in Cerebratulus (Fig. I. 

 </), where it starts at the apex of the snout on each side, and is so 

 deep as to touch the brain ; at its hindmost point the " lateral 

 canal " opens. The haemoglobin ous nerve tissue is thus brought 

 close to the surrounding medium, and on this account Hubrecht 



suggested that the cere- 

 bral organ is respiratory 

 in function. Probably 

 both the " cephalic fur- 

 row" (or cleft) and the 

 cerebral organ together 

 are derived from the 

 " ciliated pit " of Turbel- 

 laria. 



Eyes are present in 

 many Nemertines, and 

 have the structure of 

 Polyclad eyes. An oto- 



Brain of Ototyphlonemerte* (from Joubin, after Burger). 

 dr, dorsal commissure ; vc, ventral commissure ; dg, dorsal 

 lobe or ganglion ; vg, ventral ; ot, otocyst; 



Fia. XXVI. 



in 



cyst occurs only 

 lateral nerve OtotypJUonemertes, resting 

 against the ventral gan- 

 glion, and resembles the molluscan otocyst (Fig. XXVI.). 



At the anterior extremity of the snout, above the rhynchostome, 



