BY R. J. TILLYARD. 61 



or hcTemocoeie. This backward movement occurs both dorsally 

 and ventrally. The main stream runs backwards ventrally, 

 above the ventral nerve-cord, but below and latero-ventrally to 

 the alimentary canal. A smaller stream runs backwards more 

 dorsally, just below and on either side of the heart itself. 



'J'he change from backward to forward current takes place in 

 the last three segments of the abdomen, and is entirely due to 

 the suctional pull through the ostia of the heart. These ostia 

 are situated in the eighth and ninth segments. Thus some of 

 the blood will not travel ventralh^ backwards beyond the eighth 

 segment, being sucked into the ostia of that segment. Similarly, 

 some more blood will not get beyond the ninth segment. A 

 portion of this circulates only around the confines of the anal 

 area, before it returns to the ostia of the ninth segment. The 

 rest, comprised in three distinct streams, passes into the caudal 

 gills, and circulates along almost their entire length (i.p., to the 

 confluence of the two blood-canals, and back again). 



This circulation, and the vessels in which it takes place, can 

 best be seen from Text- fig. 4. In the basal piece of each gill, the 

 hfemocoele is large, and continuous with the general body-cavity. 

 As, however, it approaches the level of the breaking-joint (hj), it 

 narrows rapidly, and finally passes through the extensive hypo- 

 derm-layer at the breaking-joint as tivo mijiute canals, one situ- 

 ated dorsally, the other ventrally. On entering the gills, these 

 two canals widen out considerably, and become the closed-in 

 blood-canals of the gill. Of these two, that which carries the 

 blood into the gill maybe termed afh'ent send primary, the latter 

 term indicating (as we shall discover when we study the onto- 

 geny and phylogeny of the gill) that it was in existence long 

 before the second blood-channel was formed, at a time when the 

 efferent part of the circulation was effected (like the backward 

 circulation in the abdomen), merely along the undifiPei entiated 

 haeraoccele of the gill. The other blood-canal, which carries the 

 blood away from the gill, may be termed efferent and secondary. 

 Jn order to examine the circulation of blood in the gills, it is 

 necessary to select a transparent, lightly pigmented larva For 

 this purpose, amongst the Australian species available, the larva 



