ANATOMY AND CLASSIFICATION OF THE ARENICOLID^. 461 



sinuses becomes more and more marked, until in specimens 

 several inches long the stomach may be considered as lying 

 in a gastric sinus, from which, however, the dorsal vessel is 

 distinct from the commencement. Presumably during this 

 sinus formation the endothelial linings of the previously dis- 

 tinct capillaries unite and fuse, at least we have as good 

 evidence of their presence in the later as in the earlier stages 

 of the process.^ 



The gastric plexus consists, then, at first of a network of 

 vessels, later on of a sinus. 



Two vessels of the plexus which are ventrally situated are 

 known as the subintestinal vessels. They commence just 

 behind the heart, and run backwards to the level at which the 

 first efferent branchial vessel joins the dorsal vessel ; behind 

 this point they gradually taper and disappear. Each receives, 

 in A. marina and in A. Claparedii, six vessels, and in A. 

 cristata four vessels, from the nephridia and gills situated 

 in segments 7 — 12 and 7 — 10 respectively. The subintestinal 

 vessels are the most ventral part of the gastric plexus, and 

 they communicate, by means of the ventro-lateral vessels of 

 the plexus, with the lateral gastric vessel, and so with the 

 heart. In A. Grubii and A. ecaudata the subintestinal 

 vessels are small, and receive no vessels from the nephridia 

 or gills. 



The gastric vessel receives blood from the dorsal vessel 

 and from the subintestinal vessels by means of the branches 

 of the gastric plexus. It is usually first distinguishable pos- 

 teriorly about the middle of the length of the stomach, and 

 becomes more clearly differentiated as it proceeds anteriorly. 

 It opens into the " auricle," which is a thin-walled expansion, 

 probably of the gastric vessel. After giving off the lateral 

 oesophageal vessel the auricle opens into the ventricle, the 

 walls of which are muscular, and by their contraction drive 

 the blood into the ventral vessel. 



The lateral oesophageal vessel gives off a moderately 



' Some authors (Fauvel, 1899) deny the presence of any endothelium, and 

 state that the gastric sinus exists as such from its formation. 



