24 WALTER K. FISHER, 



and a small sinus on each side, which follows the pallial nerves for 

 a short distance and then goes directly to the mantle. These sinuses 

 are of small calibre and lead through the tissue of the spindle muscle 

 to the venous circulation of the mantle (Fig. 19 x). 



The pedal artery maintains its definite walls within the buccal 

 sinus and through its course in the foot. After leaving the anterior 

 aorta it bends sharply backward, passes under the commissure uniting 

 the two anterior enlargements of the pedal cords (pedal ganglia), 

 plunges into the tissue of the foot, and immediately divides into two 

 branches which pursue a course directly beneath the neural arteries 

 surrounding the nerve cords (Fig. 27 Ped.A, in red). The dorsal 

 wall of the pedal arteries, forms the ventral wall of the neural. The 

 relative position of the two may be seen in Fig. H. There seems to 

 be no direct communication between the two, notwithstanding the in- 

 timate association, because in many specimens, in which the neural 

 arteries had been injected from the head sinus, there occurred no 

 passage of the injection mass (which was fluid enough to follow the 

 finest vessels to the edge of the foot) into the pedal arteries. In 

 fact it is a difficult matter to inject the pedal arteries in the fresh 

 animal, as most of the material follows the cephalic or escapes into 

 the buccal sinus and injects the visceral and neural arteries. The 

 pedal artery sends a variable number of branches toward the edge of 

 the foot (Fig. 27, red) and also toward the center. These branches 

 redivide and the twigs inosculate in several places. The pedal arteries 

 and branches supply the deeper layers of the foot. 



The neural arteries, of which mention has already been made, 

 are spaces around the two great nerve cords of the foot, and are 

 homologous apparently, with similar arteries in the chiton, and in 

 simple Prosobranchs (Haliotis and Lucapina for instance). The neural 

 arteries lead out directly from the buccal sinus, and are very spacious. 

 It is probable that considerable blood circulates through this system. 

 Abundant branches follow all the larger nerves which leave the nerve 

 cord, both toward the edge of the foot and mesially. These inosculate 

 in a fine network, lying in several planes, so that is difficult to re- 

 present the system adequately. Some of the branches pass directly 

 into known venous sinuses while others become mere capillaries before 

 losing their identity. The neural arteries with branches supply more 

 particularly the upper layers of the foot, and the shell muscle. I 

 could not demonstrate in the larger branches any connection with 

 those of the pedal artery. Such a communication may occur between 

 the smaller twigs (Fig. 27 Neu.A, uncolored). 



