SECT, viii THE CIRCULATORY SYSTEM 119 



body. This is in keeping with our general explanation 

 of the morphology of the body of Apus. As we go 

 from before backwards, the organs are less and less 

 developed, the limbs are more and more rudimentary, 

 and the musculature less and less specialised ; the 

 nervous system ceases where the rudimentary limbs 

 cease, and the genital organs gradually diminish in 

 size and development. The heart is no exception ; it 

 tapers away in about the tenth or eleventh segment 

 into a point, not being developed in the more larval 

 segments which come behind. 



We may also perhaps mention that the blood in 

 many, if not in all Apodidae is coloured red, as is the 

 case in many Annelids. The direction of the blood 

 through the body is the same as in the Annelids 

 dorsally from behind forwards, ventrally from ^before 

 backwards. On its way back through the intestinal 

 sinus, which will be presently described, it streams out 

 ventrally through fenestrae in the membrane which 

 forms the sinus. (See Fig. 14, ;;/, p. 59.) It is thus 

 enabled to stream over the ventral cord, and then 

 outwards on each side along the ventral side of each 

 limb. At the end of the limb it turns round to run 

 back along its dorsal edge and thus passes through 

 the gills. It passes up thence through the lateral 

 dermo-muscular sinus of each segment into the peri- 

 cardial sinus, and thence through the ostia into the 

 heart. 1 



We have now to try and trace the origin of the 



1 As this account differs from that of Zaddach, a fuller description 

 with illustrations will be given in Appendix III. p. 296. 



