THE HABITS AND STRUCTURE OF ARENICOLA MARINA. 101 



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 intestine and 

 stomach are enclosed in a blood sinus, thickened along certain lines 

 which have been call the dorsal, gastric, and subintestinal " vessels." 

 We are, however, of the opinion that the so-called sinus is a close 

 plexns 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 subintestinal 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. hat.), which, after giving off a 

 stout branch to the oesophageal pouches, runs forwards to the buccal 

 mass, supplying the wall of the oesophagus, 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 around 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 the vessel running in the second diaphragm (3) from a vessel 

 just behind the third diaphragm (4 and 5) from the vessels to the second 

 and third nephridia. Near the middle of each somite the two neural 

 vessels are united by cross connections, which also supply the nerve- 

 cord (Pis. VIT. and V1IT, Fig. 13, N.V., X.C.V.\ 



Behind the third diaphragm the neural vessels supply the oblique 

 muscles by branches which run the whole length of the bands, and are 

 connected with the outer longitudinal parietal vessel (Fig. 13). 



