PATELLA. 243 



pressure. It is impossible to say whether the circulation 

 in the head is a loop system, or whether it is merely an 

 enlargement of the great blood space around the 

 odontophore — probably both interpretations are to some 

 extent correct, and, if so, the foot is partly supplied with 

 blood that has already been around the head. Possibly, 

 however, the blood in the head becomes re-purifled where 

 it runs near the surface in places covered by delicate 

 skin {e.g., tentacles, etc.). 



From the anterior end of the perivisceral sinus some 

 blood goes into a network of spaces in the nuchal mantle, 

 and these lead oif ultimately into the auricle through 

 the linear series of small apertures seen on opening the 

 latter. This blood must be partly oxygenated. Blood 

 channels from the mantle, in more primitive forms, 

 probably opened, as they do in the Rhipidoglossa, into the 

 efferent ctenidial veins, but as these latter disappeared 

 they have become directly connected with the auricle, and 

 have undergone compensating development. 



The blood reaching the left kidney, in a more primitive 

 form, would have gone out to the corresponding ctenidium 

 for oxygenation, returning thence by the left branchial 

 vein to the left auricle. A change has necessarily had 

 to follow the disappearance of the left ctenidium, and 

 now in Patella there is some doubt as to the circulatory 

 arrangements of the left kidney. Possibly it receives 

 blood from the perivisceral sinus and passes it on almost 

 direct to the auricle. If this is not the case, this organ 

 must be in direct dependence on the auricle for its blood 

 supply — a condition which its honiologue in Haliotis seems 

 to have attained. The circulatory arrangements of the 

 left kidney, even of the primitive Patella, show how far this 

 organ has departed from the ordinary condition of a kidney 

 among the loVer 'Gastropods. 



