THE COMMON MUSSELS. 23 



arteries do not branch greatly. But on the smooth sides of the palps 

 the tentacular artery runs very near the anterior edge, and sends back 

 on its posterior side numerous sub-parallel short arteries, the posterior 

 ends of which branch and link with their neighbours, forming a kind 

 of plexus on the anterior half of the smooth surfaces of the palps 

 (Figs. 27 and 28). The surfaces having such a plexus are the dorsal 

 side of the outer palps and the ventral side of the inner palps ; the 

 sides of the palps that face one another have a single artery. There 

 are then in all eight tentacular arteries, two to each palp : the upper 

 six of these come from the transverse artery in the floor of the 

 supra-sesophageal cavity, while the two lower come from the anterior 

 ventral artery. In one specimen the dorsal tentacular artery of 

 the inner palp was also derived, as the ventral one, from the anterior 

 ventral artery. In this case the outer and inner palps derived their 

 blood-supply from different directions. 



Arteries to the muscles and the foot. The artery in M. latus that 

 supplies the anterior retractors, the foot, and the apices of the mus- 

 cular V s formed by the middle and posterior retractors of the byssus 

 springs, as already mentioned, from a branch of the anterior aorta, 

 and is the anterior ventral artery. The anterior ventral artery passes 

 down just in front of the anterior coil of the intestine, and plunges 

 between the anterior retractors. It turns back between the retrac- 

 tors, giving off arteries to them, also a tentacular artery forward, and 

 a pedal artery to the anterior side of the foot. It passes above the 

 apex of the V formed by the anterior retractors, then under the pedal 

 ganglion, and sends arteries to the base of the byssus and between the 

 bases of the middle and posterior retractors, supplying then the whole 

 muscular floor formed by the anterior retractors, the foot, and the 

 byssus- [p «, Figs. 24 and 28). 



The portions of the middle and posterior retractors near their in- 

 sertion on the shell are supplied by small arteries from the posterior 

 aorta, or some of its branches. The posterior adductor is supplied by 

 a large artery (Fig. 24, B) from a branch of the posterior aorta, 

 descending in front of it (Fig. 24) . 



Arteries to the mesosoma, reproductive organs, fyc. The mesosoma 

 is supplied by the posterior ventral artery (p v a, Fig. 24), whose 

 lateral branches supply the portions of the reproductive organ in its 

 neighbourhood. The parts of the reproductive organ below the peri- 

 cardium, and not superficial, are supplied by branches of the posterior 

 aorta, and of the descending artery in front of the posterior adductor. 

 The superficial parts of the reproductive organ and those contained in 

 the mantle-lobes are dependent on the pallial arteries for their blood- 

 supply. 



