SECT, vin THE CIRCULATORY SYSTEM 121 



tube running through the whole trunk outside of the 

 dermal musculature, and containing the intestine and 

 the genital glands, its origin is at once suggested to 

 us. It appears to be the parietal layer of the coelom 

 epithelium of the original Annelid, loosened from the 

 body wall except at certain definite points, viz., 

 where it is in contact with the ventral muscle bands, 

 and laterally along the segmental constrictions, where 

 it remains attached by means of the septa above men- 

 tioned, each of which extends dorsally as far as the 

 point of attachment of the dorso-ventral muscle 

 bands. (See Figs. 66, p. 297, and 67, p. 298.) 



In this way we should at once get just such an 

 intestinal sinus as we find in Apus, viz., a membra- 

 nous tube lying just inside of the dermal musculature, 

 the transverse dorso-ventral muscles being almost the 

 only muscles found within the tube. Indeed the rela- 

 tion of the membrane to these muscle bands seems to 

 support this view, for where these arc attached to the 

 dorsal surface, the membrane itself is raised up into 

 conical folds in the manner illustrated in the diagram 

 (Figs. 14, p. 59, and 66, /). This certainly looks as if 

 the membrane had, as it were, fallen away from the 

 integument. If this view is correct, the intestinal 

 sinus corresponds with the body cavity of the original 

 Annelid, and the dermo-muscular sinus of each seg- 

 ment is a new formation caused by the loosening of 

 the epithelium from the body wall. 



The dissepiments themselves may be folds of this 

 membrane grown together. If so, these partial dis- 

 sepiments must have been secondarily acquired, after 



