NEMERTEANS 



227 



The vascular system is first seen in serial sections as a single relatively large lacuna, 

 with strands of tissue passing across it vertically, lying above the commencement of the 

 rhynchodaeum at the tip of the head. Farther back the intrusion of the rhynchodaeum 

 causes this lacuna to be divided into two lateral and a median lacuna. The latter has a 



Fig. 2. Tubulanus nothus, Burger. A, transverse section of head at anterior end of mouth; 

 B, transverse section of body at nephridial canal. 



narrow lumen and lies above the rhynchodaeum. Where the pharyngeal nerves leave 

 the brain the lateral blood spaces are joined beneath the rhynchodaeum by a ventral 

 connecting lacuna and at the same place the laterals are again united with the median 

 dorsal. The latter shortly afterwards disappears. Immediately 

 behind the ventral connecting blood space a pair of small 

 vessels separate off" from the laterals. These remain close to 

 the angle of the pharynx, but at the posterior end of the 

 mouth they rejoin the laterals. There are subsequently one 

 or two downward extensions of the laterals forming small 

 loops (Fig. 3). I could not trace the close connection of the 

 vascular system with the excretory tubules described by 

 Oudemans (1885) in Carinella annulata. The excretory 

 tubules occur above the lateral vessels and open to the ex- 

 terior by a single duct high up on the dorsal surface 

 (Fig. 2B). 



The nervous system lies between the basement membrane 

 and the circular muscles. The brain consists of two lateral 

 curved plates of tissue with no observed demarcation be- 

 tween cellular and fibrous tracts. The plates are united by grap b hic reconstruction showin 

 a stout ventral commissure, and a little farther back by a the vascular system (dorsal). 

 slender dorsal commissure. There is little differentiation 



into dorsal and ventral ganglia until the posterior end of the brain. Anteriorly the brain 

 breaks up into many nerves to serve the tip of the head. Just in advance of the ventral 

 commissure a pair of nerves is given off for the proboscis. Another pair originates from 

 the ventral ganglia after the commissure and innervates the pharynx. A mid-dorsal nerve 

 intersects the dorsal commissure. It passes down the body between the basement mem- 



Fig- 3- 

 Burger. 



Tubulanus nothus, 

 Diagram from a 



