49 



and then goes, on in front of tlie rectnm, soon curving 

 downwards and forwards until it becomes continuous 

 witli an anterior sinus, surrounding- at least the front 

 part of the radular sac, and enhirging forwards so as to 

 enclose the ciishion of the odontophore. 



The movements of the odontophore must affect this 

 sinus, especially as it connects with the pedal sinuses 

 and other blood channels. It is in this way that these 

 latter sinuses probably got blood pumped into them. 

 This opinion is grc^atly strengthened by the observation 

 of the head of living specimens, in which the odontophore 

 can sometimes be seen moving forwards and backwards 

 almost rhythmically. It is interesting to note that a 

 similar and similarly connected sinus exists in Chiton, 

 and the pumping work thus done outside the heart 

 may be correlated with the feeble muscularity of the 

 ventricle in so many of the lower Molluscan forms. The 

 existence of arterial branches of the aorta, other than 

 the posterior or visceral artery, is doubtful, though 

 possibly one goes into the salivary glands. 



The Visceral Artery seems to branch almost imme- 

 diately after leaving the left posterior corner of the 

 pericardium, one branch certainly goes to the gonad 

 region and the other also seems to enter the visceral 

 hump. 



The course of the arteries cannot be traced far from the 

 pericardium, the greater part of the blood system being 

 lucunar, i.e., consists of spaces which have not a definite 

 epithelial lining. 



Lacunce and Sinuses (Blood Sjyaces). — Blood from the 

 Anterior Aorta goes into the great anterior sinus sur- 

 rounding the radular sac and odontophore cushion. 

 Thence it proceeds into 



(rt) The Pedal Sinuses (fig. 8a). — These are fairly 



E 



