HABITS AND STRUCTURE OP ARBNIOOLA MARINA, 17 



whicli are also found in older specimens on the vessels running 

 to the body-wall. This chlorogogenous tissue is first seen o.i 

 the ventral vessel about the level of the eighth pair of sp*8e. 

 In the tail the ventral vessel ends in the obliquely p'aced 

 intestinal vessels which encircle the intestine, and which form, 

 along with the capillaries from its median terminal portion, 

 the commencement of the dorsal vessel. 



Visceral Plexus.— Wiren (1887) maintains that the in- 

 testine and stomach are enclosed in a blood-sinus, thickened 

 along certain lines which have been called the dorsal, gastric, 

 and subintestinal " vessels." We are, however, of the opinion 

 that the so-called sinus is a close plexus of vessels, some of 

 which appear to have a distinct cellular lining. The dorsal 

 vessel is, at any rate, a perfectly distinct structure with 

 proper walls. 



The subintestinal vessels (fig. 5, S. V.), which commence 

 just behind the heart and run backwards, are moderately large 

 up to the level of the thirteenth setse, but then taper rapidly 

 and gradually disappear. They each receive seven segmental 

 vessels. The first of these comes from the fourth nephridium, 

 the second from the fifth nephridium and the first gill, the 

 third from the sixth nephridium and second gill, and the other 

 four from the third, fourth, fifth, and sixth gills. The sub- 

 intestinal vessels open . through the plexus into the lateral 

 gastric ones, and so into the heart. The flow in these vessels 

 is probably slowly forwards. 



The gastric vessels give off from the " auricle," into which 

 they expand, a lateral oesophageal vessel {Oe. Lat.), which, after 

 giving off a stout branch to the oesophageal pouches, runs 

 forwards to the buccal mass, supplying the wall of the oeso- 

 phagus, as it does so, with numerous small branches. 



Neural Vessels. — These are a pair of small vessels lying 

 one on each side of the ventral nerve-cord, and accompanying 

 it from one end of the body to the other. They arise round the 

 nerve-connectives from the brain from capillaries of the dorsal 

 vessel, and receive several branches from the ventral vessel (1) 

 midway between the first and second diaphragms, (2) from 



VOL. 41, PART 1. NEW SER. B 



