HEKATEEOBRANCHUS SHEUBSOLII. 183 



(figs. 2 and 12, d.v.) in the anterior region of the body, con- 

 tinued forwards into the prostomium. Just before it reaches 

 the prostomium two vessels are given off, one to each branchia 

 {d. hr. v.). These run up the inner sides of the branchiae, and 

 return by vessels on the outer side («. hr. v.), which meet in 

 the median line on the ventral surface in the posterior part of 

 the first segment to form the ventral vessel {v. v.)} Before they 

 meet each appears to give off or be joined by the single con- 

 tractile vessel going to the ''cephalic" tentacle {t.v.). The 

 ventral vessel runs throughout the whole length of the body 

 (figs. 2, 5, 6, 7, 8, V. y.), passes in the anal segment into a 

 sinus (figs. 2, 7, 8, sin.) surrounding the intestine, and lying 

 just outside the epithelium, probably between it and the cir- 

 cular muscular layer; or it may be that the circular muscular 

 layer is really absent in this region, and that the sinus lying 

 between the intestinal and the ccelomic epithelium of the ali- 

 mentary canal has some contractile power of its own, as it 

 has in other sedentary annelids, e. g. Spirographis,^ where, 

 however, muscular fibres are present as well. This sinus com- 

 pletely surrounds the intestine in the whole of its posterior 

 non-constricted part, and is at first continued over part of the 

 constricted part ; then, however, a nucleated mass appears 

 inside it on the median dorsal line of the wall of the intestine, 

 and forms a longitudinal upstanding ridge. Part of the sinus 

 closes in round this ridge, and becomes nipped off from the rest 

 of the sinus (figs. 2 and 6, d. s. v.), and so is continued forwards 



and what is given in the text is only what appears to me — after the examination 

 of numerous living specimens and series of sections — to be its probable 

 distribution. When living the animal is too opaque, when dead the vessels 

 are seldom in the same state of contraction or expansion in two individuals. 

 The vascular system in the highest animals even is subject to individual 

 variation, and it may be that there are really slight individual variations in its 

 arrangement in worms, and in this amongst others. 



' The direction in which the blood flows in the branchise cannot be deter- 

 mined, as the vessels cannot be seen in the living. It probably may flow in 

 either direction, from the ventral to the dorsal at one time and from the 

 dorsal to the ventral at another. 



* Claparede, 1873, ' La Structure des Aunelides sedentaires.' 



