STRUCTURE OF ARENICOLA. i8i 



the gut. Two sub-intestinal vessels lie between the ventral 

 vessel and the gut, and receive tributaries from the anterior 

 gills. On each side of the digestive part of the gut there is 

 a lateral vessel. 



Just behind the posterior pair of oesophageal glands lies a 

 very contractile heart. It consists of two lateral chambers, 

 or ventricles, each of which receives blood from the dorsal 

 vessel, from a sub-intestinal vessel, and from a lateral vessel, 

 and drives blood into the ventral vessel. Each of the lateral 

 vessels before entering the heart expands into a kind of 

 auricle. 



The longitudinal vessels are all connected by transverse 

 branches. From the ventral vessel arise afferent branchial 

 vessels. From the seven posterior gills efferent branches 

 enter the dorsal vessel ; while those from the six anterior 

 gillS join the sub-intestinals. Each efferent vessel gives oft 

 a branch to the skin, while the dorsal and sub-intestinal 

 vessels give off numerous branches to the walls of the gut. 

 It seems that the flow of the blood is not always quite the 

 same. 



Respiratory System. — There are thirteen pairs of gills. 

 Each is a tuft of thread-like branches, through the thin walls 

 of which the red blood shines. As the papillae on the 

 proboscis are hollow and contain vessels, they are doubtless 

 of respiratory significance. Indeed, the gills may be regarded 

 as exaggerated papillae. 



Excretory System. — In the anterior region, from the 

 fifth to the tenth segments, there are six pairs of nephridia. 

 Each consists of three parts — a funnel opening into the 

 body-cavity, a glandular portion, and a bladder communi- 

 cating with the exterior. 



Reproductive System. — The sexes are separate and similar. 

 The reproductive organs are very simple modifications of the 

 peritoneal membrane of the body-cavity. They are developed 

 in close association with the nephridia. The reproductive 

 cells are liberated into the body-cavity, and there matured. 

 In August and September they pass out by the nephridia. 

 Nothing certain is known in regard to the development. 



