504 Cresswell Shearer 



Ikeda lias shown tliat it is trausformed later into tlic v'mg vessel 

 of the adult. De Sely.s Longciiamps has moreover made it clear, 

 that this cavity is distinctly 1)la8tocoelic. It has therefore nothing 

 to do with the true coelom, hat is })robably formed from the rem- 

 uants of the blastocoelic spaee iu the trimk region not taken up hy 

 the true eoelom, which is tlie spaee surroundiiig- the line of iusertion 

 of the tentacles, into which it extends a short distance. Its formation 

 takes place I think in the follo wing manner. As the coelomie sac 

 forms and grows roinid the gut, and at the same time spreads on 

 to the posterior portion of the stomach, it will be seen from the 

 iuspectiou of pl. 31 fig. 18, that the original blastocoelic space of 

 the larva is confined to the tentacles, and the part of the trunk region 

 about the stomach. Now shortly after this, the anal region Starts 

 to grow much more rapidly than the rest, so that the anal papilla 

 containing the gut and coelom is drawn out, so that this part of 

 the larva comes in time to be the largest. The part of the blastocoel 

 surrounding the base of the tentacles thus comes to occupy an 

 anterior instead of a posterior position in the larvai trunk. When 

 the fii.Uy formed Actinotrocha stage is reached, this space is seen 

 surrounding the larva like a collar at the base of the tentacles, 

 where it retains its position until transformed into the adult ring 

 vessel during metamorphosis. It is piain tliis cavity is not 

 truly coelomie, the epithelial lining on the stomach is always 

 imperfect, and numerous waudering cells found throughout it denote 

 its haemocoelic nature. The septum between this cavity and the 

 trank region is formed by the anterior wall of the true coelom of 

 tlie trunk region which has moved forward. 



Contrary to the opinion of De Selys Longchamps I have tried 

 to show that the trunk coelom and the cavity of the collar are never 

 in communication , as the true coelom of the trunk is a closed sae 

 from the first. Late in devclopment only does the true coelom of 

 tlie collar region form, which Mastermax never observed, and this 

 during metamorphosis is transformed into the adult supraseptal 

 cavity. The origin of this cavity is very difficult to observe, in 

 all the material I have been able to collect it is unformed and 

 only in the late stages of the Actinotrocha is it well established. 

 As far as my observations go they would seem to strecgthen the 

 statement of Schultz (21) that this cavity is derived by segmen- 

 tation from the ventral anterior prolongations of the trunk 

 cavity into the collar region. It is hard to see why it should 



