NOTES ON COMMON SPECIES OF TROCHUS. 467 



surrounding the pedal ganglia and pedal cords. Thence 

 blood is distributed throughout the foot. It appears to 

 gather again in a median sinus above the nerve-cords, which 

 opens in front into the general cavity of the head. The blood 

 iu the head is thus in part returned from the foot, and in 

 part supplied from the branch of the aorta which surrounds 

 the radular sac. The muscles of the odontophore are probably 

 mainly supplied from the latter channel, while the blood from 

 the former source is very likely partly aerated in the head 

 when the animal is extended and active. 



The blood from the general cavity of the head is collected 

 into a fairly definite channel which carries it bach to the 

 right kidney. These species of Trochus, however, differ 

 from Haliotis in that the anterior lobe of this kidney is much 

 reduced, so that the blood channel does not run nearly all 

 the way iu the kidney wall, as is the case in Haliotis. The 

 right kidney also receives blood from the visceral hump 

 further back. In fact the blood flow from the head is by no 

 means so intimately connected with the right kidney as in 

 Haliotis. That organ must, in Trochus, be mainly a purifier of 

 blood from the visceral hump. 



An afferent channel from the right kidney takes blood from 

 these two sources into the roof of the branchial cavity at the 

 back, and runs across beneath the rectum to the left kidney, 

 but first gives off a channel into the branchial roof. We 

 look upon this last channel as a probable homologue of the 

 afferent channel of the right ctenidium of Haliotis which has 

 been lost in Trochus. The efferent channel from the right 

 kidney thus corresponds with a part of the basi-branchial 

 sinus of that type. 



The blood reaching the neighbourhood of the left kidney, 

 as just described, goes iu part to that organ, but the main 

 flow runs forward in a subrectal sinus, and then turns to the 

 left, crossing the roof of the branchial cavity, as the trans- 

 verse pallial vein of most authors (figs. 4 and 5). 



Before reaching the left kidney^ however, it communicates 

 with the right auricle, which also receives a channel from the 



