184 FLORENCE BUCHANAN. 



on the intestine, the ridge inside it being separated from the 

 intestinal epithelium by a very fine layer of cojlomic epithelium 

 only. Some series of sections would seem at first sight to 

 show that the ridge was in its posterior part directly continuous 

 with the intestinal epithelium ; but a more careful examina- 

 tion leads rather to the conclusion that it is formed by the 

 tucking-in of the coelomic epithelium which lies outside the 

 sinus on either side. It lies, however, especially posteriorly, 

 exceedingly close to the intestinal wall. Its significance 

 (whether physiological or morphological) is as difficult to deter- 

 mine as that of the so-called" Herzkorper" or "cardiac body" 

 of certain other Polychsets,^ which it in all probability repre- 

 sents. No lumen is to be seen in it here throughout its course. 

 In the oesophageal region the nipped-off upper part of the 

 sinus enclosing the longitudinal ridge (fig. 2) leaves the walls 

 of the alimentary canal, and becomes the contractile dorsal 

 vessel which runs upwards until it comes to lie just beneath 

 the thin part of the body-wall in the dorsal median line, i. e. 

 where the longitudinal muscle layer is only very feebly deve- 

 loped (fig. 5,d.D.). It is here surrounded by a well-developed 

 circular muscular layer {c. m'.) to which its contractile power is 

 due. The walls of all the other vessels and of the sinus appear to 

 consist only of coelomic epithelium. It is difficult to say what 

 happens to the rest of the sinus (which is continued throughout 

 the intestinal region) when the dorsal vessel finally leaves the 

 wall of the alimentary canal in the oesophageal region. It 

 certainly is not continued as a sinus, but whether it forms vessels 

 or not is a difficult point to determine, since there are other very 

 much coiled vessels in each segment of the oesophageal region. 

 These coiled transverse or dorso-ventral vessels seem to me to 

 connect the dorsal and ventral vessels (as shown diagramma- 

 tically in fig. 12), but it may be that they connect the ventral 

 not with the dorsal, but with lateral vessels which are continua- 

 tions forwards of the sinus, lying, for some part of their course 

 at least, close to the dorsal vessel. The dorso-ventral vessels 



1 See Cunniugham, " Some Points iu the Auatomy of Tolycbaita," thia 

 Journal, vol. xxviii, 1887. 



