122 THE APODID^: PARTI 



the original Annelidan dissepiments had disappeared. 

 There is no great difficulty nor, indeed, improbabi- 

 lity in such a supposition. We may indeed find in 

 these dissepimental folds, attaching the membrane 

 to the body wall, traces of the former presence of 

 the old Annelidan septa, which may have originally 

 run in between the folds just as the transverse dorso- 

 ventral muscles run up into the conical folds of the 

 membrane, as already described and marked / in the 

 figures. 



If this latter view is correct, we then explain the 

 origin of the lacunar system of Apus by the loosening 

 of the ccelom epithelium from the heart and from the 

 dorsal surface in the immediate neighbourhood of the 

 heart so as to form the longitudinal cardial sinus. 

 On each side of this sinus laterally it remains attached 

 to the intersegmental folds, being loosened, however, 

 from the segmental walls, so as to form the lateral 

 segmental dermo-muscular sinuses, which conduct the 

 blood from the gills to the cardial sinus. 



The separate stages by which this lacunar system 

 took the place of a blood vascular system, are not 

 difficult to imagine. The first step in the transforma- 

 tion would be the gradual degeneration of the separate 

 blood-vessels and the consequent mingling of the 

 blood with the body fluid or lymph. The disappear- 

 ance of the vessels supplying the dermal musculature, 

 and the general diffusion of haemolymph between 

 the integument and the ccelom epithelium might 

 very easily lead to the loosening of the latter from 

 the integumental musculature, excepting along 



