TWO AQUATIC OLIGOCIOETES. 25 



ii). The integumentary vessels are always present in the 

 posterior segments. They branch out from the dorsal vessel at 

 the posterior part of a segment, run toward the ventral side, and 

 immediately behind the ventral setae they penetrate into the 

 hypodermal layer of the body wall, in which they take an un- 

 dulating course and come out again into the ccelom at the same 

 point where they entered it, and then proceed straight to the 

 ventral vessel into which the} 7 open. 



In the posteriormost segments of the body these integument- 

 ary vessels are absent and the two or more commissural vessels 

 form a large ring in the ccelom. 



I have sometimes found a blood sac (fig. 10, bs) in both 

 Japanese and Indian specimens. It lies on the dorsal side of 

 the dorsal vessel in segment I, and is connected with the latter 

 close to the origin of the prostomial nerves. 



5). The contractile hearts lie in segment VIII, and originate 

 from the supra -intestinal vessel between the middle and last third 

 of the segment (fig. 14). They take a winding backward course 

 on either side of the intestine, and on reaching the posterior 

 septum, they make a sudden turn towards the ventral vessel and 

 open into it. Sometimes they run parallel to and near the 

 ventral vessel to the middle of segment IX, passing on the way 

 through the septal sacs of VIII/IX. 



The wall of the contractile heart is very 

 thick compared with that of the ordinary vessels. 

 The outermost layer of the wall is the perito- 

 neum, and inside of this lies a curious layer 

 consisting of bundles of exceedingly fine, long 

 fibres, which take up the fuchsin of Malloey's 

 stain as well as eosin. They also stain deeply 

 with Heidenhain's haematoxylin. I can not 

 understand their nature. This layer is sometimes 

 penetrated by intrusions of the peritoneal cells, 

 Fig. 17. which may possibly contain nerves. 



l gotoi Part of Next to ^q above mentioned layer comes 



the heart wall in ^ 



cross section. the circular muscle layer, which is well developed ; 



