468 H. J. FLEURE AND MURIEL M. GBTTINGS. 



branchial roof. Following Thiele^ we homologise the latter 

 channel with the efferent vein of the right ctenidium (lost in 

 Trochus) of Haliotis, 



In Haliotis the right auricle is supplied from the right 

 efferent ctenidial vein^ and also connects with the basi- 

 branchial sinus, so conditions are essentially similar in 

 Haliotis and Trochus, allowance being made for the loss of 

 the rio'ht ctenidium in the latter. 



The channel from the right to the left kidney in Trochus 

 together with the forwardly running subrectal sinus and the 

 transverse pallial vein, may be homologised with the basi- 

 branchial sinus of Haliotis. The changes in detail are con- 

 nected with the large size of the left kidney and its position 

 in the roof of the branchial cavity. 



As already stated, the blood spaces beneath the epithelium 

 of the left kidney are supplied from the channel arriving 

 from the right kidney, and so some of the blood (particularly 

 that from the visceral hump) may be purified from nitrogenous 

 waste. That from the head is, however, by no means so com- 

 pletely purified as it has really only skirted the right kidney. 



The transverse pallial vein shows interesting differences in 

 the two species considered. It is equally obvious on dissec- 

 tion in both, mainly owing to the covering ridges of the 

 mucous gland, but the blood space is relatively larger in 

 T. obliquatu s, Gmel., than in T. crassus, Mont. In the 

 former, also, its anterior and posterior branches are essentially 

 afferent ctenidial channels sending blood through the gill 

 axis into the leaflets. In T. crassus the gill does get some 

 blood in this way, but a great part of the contents of the 

 transverse pallial vein goes out into the roof of the branchial 

 cavity which also receives blood from the sub-rectal sinus. 

 There seems, therefore, ground for the opinion that the roof 

 of the branchial cavity in this mid- to high-tide form is of 

 considerable importance in respiration, while the ctenidium 

 cannot have quite so much importance as in the other 

 species. 



The blood from the ctenidium together with that brought 



