COE : NExMERTEANS OF WEST AND NORTHWEST COASTS. 41 



mentary canal. The system has been studied in detail by von Ken- 

 nel ('78), Takakura ('97), and Maclaren (:01). 



D. Heteronemertea. — In this order the three longitudinal ves- 

 sels of the hoplonemertean are always present, and branch freely, 

 forming numerous anastomoses and large lacunae. There are also 

 esophageal and dorso-lateral vessels present in the esophageal region 

 in many forms, and in some species of Cerebratulus rhynchocoel 

 vessels likewise occur. The lateral vessels are usually very large in 

 the esophageal region and lie in the angle between the esophagus 

 and the proboscis sheath, but become of smaller size in the intestinal 

 region and are situated ventrall}^, often approaching medially nearly 

 to the axial line. 



The dorsal vessel arises from a broad anastomosis of the lateral 

 vessels (usuall}^ dilated to broad lacunae) in the brain region, jDasses 

 into the rhynchocoel, as in the Hoplonemertea, and remains in this 

 cavity throughout nearly or quite the entire length of the esophageal 

 region, behind which it lies beneath the proboscis sheath. It has 

 metameric anastomoses with the lateral vessels, as in the Hoplone- 

 mertea. 



The cephalic vessels dilate to form very large lacunae, united 

 anteriorly, surrounding the rhynchodaeum. Occasionally they are 

 represented by numerous anastomosing vessels of small size. The 

 lateral vessels, or lacunae, cover the medial faces of the brain lobes 

 and surround the cerebral sense organs on all sides except anteriorly. 



The esophageal vessels often branch profusely, and are often 

 indistinguishable from ramifications of the similarly branching lateral 

 vessels. Both sets of tubules may enter into close contact with 

 the fine branches of the nephridial system. 



The rhynchocoel vessels when present are similar to those of 

 Carinella and are limited to the eso])hageal region. 



In those forms having a caudal cirrus (Zy(5eupolia, Micruba, 

 MiCRELLA, Cerebratulus) the lateral vessels continue into this 

 organ, sometimes forming a single large central blood space (Text- 

 fig. 4). 



Histological structure. — The blood vessels consist of a rather thick 

 and firm basement layer thrown up into conspicuous ridges when 

 contracted, lined internally \Wth a very thin endothelium with minute 

 oval nuclei, and surrounded by a layer of extremely hue circular 

 muscular fibers. Outside the muscular layer, where the vessel is 



