.I20 THE APODID^ PART i 



lacunar system of Apus more in detail. Its main 

 features are very simple, and in this respect it shows 

 a primitive character. In the main it may be said to 

 consist of but one membrane, forming a tube which 

 runs from the anterior end of the first trunk segment 

 (where it is attached all round to the body wall) to 

 the end of the body. This membranous tube sur- 

 rounds the intestine and genital glands, while between 

 it and the body wall lie the heart, the ventral cord, 

 and all the musculature, except the dorso-ventral 

 bands which run between the intestine and the genital 

 glands. (See Fig. 14, p. 59 ; cf. also Fig. 66, p. 297.) 

 The membrane is attached to the body wall, at least 

 in the first eleven trunk segments, by segmental dis- 

 sepiments, which correspond with the segmental con- 

 strictions of the body. These dissepiments extend 

 dorsally to the points of attachment of the dorso- 

 ventral muscles, which raise up the membrane in 

 conical folds. Between these dorso-ventral muscle 

 rows the membrane hangs free of the dorsal body wall, 

 thus forming the cardial sinus, in which the heart is 

 expanded by an arrangement of connective tissue 

 fibres. 



In trying to trace the origin of this membrane and 

 these dissepiments (see Fig. 6j, s, p. 298) from the 

 internal organs of the original Annelid, we naturally 

 begin with the latter, as reminding us at once of the 

 Annelidan septa. Are they the remains of such 

 septa ? The answer depends on the interpretation we 

 give to the membrane forming the intestinal sinus. 



Glancing at the membrane then as a whole, as a 



