48 bulletin: xMuseum of compaeative zoology. 



chyma entirely internal to the musculature of the head (Text-fig. 19; 

 PI. 19, fig. 119) and in the Heteroneraertea it also lies mainly inter- 

 nal to the cephalic musculature by which it is closely surrounded. 



In all nemerteans the four lobed brain consists of two ganglionic 

 masses on each side — the dorsal and ventral ganglia. The anterior 

 ends of the dorsal ganglia of the two sides are united above the 

 anterior end of the rhynchocoel by the dorsal commissure, while a 

 considerably stronger ventral commissixre ^^nites the ventral ganglia 

 in a corresponding position beneath the rhynchocoel (Text-figs. 3, 

 19; PI. 16, figs. 93, 95, 97, 99). 



In most Paleonemertea the dorsal ganglia are larger than the 

 ventral, while in the other orders the ventral ganglia are the larger. 

 The apparent size of the dorsal ganglia in the Heteronemertea, how- 

 ever, is usually greatly increased by a close fusion with the cereljral 

 sense organs which commonly assume the appearance of posterior 

 lobes of this portion of the brain. Here the dorsal ganglia are 

 bilobed posteriorly, the dorsal lobe ending blindly, while the ventral 

 continues directlv into the sense oro-an. 



As a rule the proboscis is attached to the wall of its sheath in the 

 immediate vicinity of the brain commissures. The blood vessels 

 connecting the cephalic blood spaces with the vessels of the rest 

 of the body also pass between the dorsal and ventral commissures. 

 In the Hoplonemertea the esophagus lies immediately beneath the 

 ventral commissure (Text-figs. 3, 19; P]. 19, fig. 119). 



The position of the lateral nerves with respect to the layers of 

 the body walls is even more variable than that of the brain. In 

 Carixina and Procarinina these nerve cords lie wholly in the 

 integument ; in Carinella they are situated between the base- 

 ment layer and the outer circvilar muscles ; in Carinoma they lie 

 outside the circular muscles anteriorly, but Avhen as far back as 

 the posterior end of the esophageal region pass through these 

 muscles and take up a position throiighout the rest of the body in 

 the midst of the longitudinal muscular layer (PL 13, figs. 81, 82). 

 In Carinomella a condition intermediate between that found in 

 Carijstella and that in Carinoma occurs, for in Carinomella 

 the lateral nerves lie outside the outer circular muscles anteriorly, 

 but more posteriorly they sink inward among the fibers of the 

 longitudinal muscles, although they still carry beneath them a 

 portion of the circular muscular fibers (PI. 8, fig. 58; PI. 9, fig. 



