22 WALTER K. FISHER, 



planation of the ctenidial vibrations is correct, we may suppose the 

 heart-beats to average about forty per minute. 



Arterial System. The ventricle opens, as stated above, 

 directly into the aorta which lies on the floor of the pericardium, 

 nearly parallel with the posterior boundary. The anterior (aorta) and 

 posterior (genital artery or posterior aorta) branches or parts form 

 essentially one continuous tube, which may be considered as simply 

 passing under the ventricle and opening without an intermediary tube, 

 directly into the ventricle. The anterior aorta (Figs. 1, 3, 21 and 24 

 Änt.Äo) after leaving the ventricle passes through the loop formed 

 by the visceral commissure, to the right of the oesophagus, into the 

 head cavity where it turns abruptly downward. Just after quitting 

 the pericardium the aorta gives off a small artery which passes over 

 the viscera and enters the floor of the rectal portion of the right 

 kidney, whence it sends branches to the ureter and finally reaches 

 the circulation of the mantle. Within the buccal sinus another small 

 branch is given ofi" to the oesophagus. Arriving just behind the buccal 

 mass (Fig. 3) the aorta divides into two branches, the pedal artery 

 (Ped.Ä) and cephalic artery {Cej)h.Ä). The latter passes along the 

 mid-dorsal portion of the buccal mass, surrounding the radula tube. 

 Almost at once a ventral branch is given off, the sublingual artery 

 (Sub.L.A) which leads directly downward between the two posterior 

 processes of the buccal mass, through the muscular tissue, and comes 

 to lie along the ventral side of the buccal mass embedded in muscle. 

 Here it soon loses its walls, and passes by rather definite spaces, 

 into the sublingual organ, palps, and a broad sinus, under the buccal 

 cartilages, by which it is in cohimunication with a similar sinus, 

 above the cartilages. The cephalic artery leads into the upper lip of 

 the radula tube thence around under the chitinous wing-like pro- 

 jections of the radula, into the definite sinus above the buccal carti- 

 lages, just spoken of. Hence the cephalic and sublingual arteries are 

 ultimately again in communication. The blood now either finds its 

 way into the buccal sinus {B. S) or goes directly by two small openings 

 under the palps, into spaces in the lips, which are protruded by this 

 medium. 



We saw that the aorta bends downward and divides into a pedal 

 and a cephalic artery within the confines of the buccal sinus or 

 head cavity. Just at this point of division the radula tube, which 

 has come backward inside of the cephalic artery, pierces the wall of 

 the artery on the left side and passes out into the buccal sinus, to 



