SEA MUSSEL MYTILUS EDULIS. 151 



immediate region, the dorsal and lateral walls of the supra-cesophageal cavity, and to 

 the anterior adductor and anterior retractor muscles. 



The anterior ventral artery is a median trunk that arises from the anterior aorta 

 ventral and slightly posterior to the point where the terminal arteries branch off. It 

 runs forward in the middle of the floor of the supra-cesophageal cavity for about half 

 its length and then turns sharply downward, crossing the oesophagus on its right side 

 and continuing to the ventral body surface between the anterior retractor muscles. 

 Here it turns backward and gives off in its course a large pedal artery to the foot besides 

 a number of smaller vessels to the anterior and posterior retractor muscles and to the 

 byssus organ. 



The tentacular arteries are two anterior branches from the anterior ventral artery. 

 The dorsal tentacular artery continues forward on the floor of the supra-oesophageal 

 chamber from the angle where the anterior ventral artery turns downward and extends 

 to the mid-dorsal region of the upper lip. There it forks into right and left branches 

 that extend laterally to the dorsal palps on their respective sides. The vessels enter 

 the basal region of the palps on their ribbed sides and pass transversely across to the 

 smooth border, which they follow back to the distal extremity, giving off at right angles 

 in their course numerous fine vessels which extend across the palp to its lower edge. 

 The ventral tentacular artery branches off from the anterior ventral artery where it turns 

 abruptly backward on the ventral side of the body. It runs forward slightly beneath 

 the ventral surface between the anterior retractor muscles to the ventral surface of the 

 lower lip, where it forks into right and left branches that go to the right and left 

 inferior palps, respectively. The course of the vessels through the inferior palps is the 

 same as that already described for the superior palps. 



VENOUS SYSTEM. 



The venous system collects the blood of the body into the 10 main groups of vessels 

 through which it is conveyed to the heart. Briefly described, they are as follows: 

 (1) A marginal sinus which extends around the border and receives the blood of the 

 mantle, (2) a large number of ascending pallial veins on the inner face of the mantle 

 which collect the blood of this organ, (3) a pair of horizontal veins which extend the 

 length of the mantle just below the roots of the gills and receive the blood from the 

 ascending pallial veins, (4) a pair of large intermuscidar veins in the region of the muscles 

 of the foot and byssus, (5) a pair of mesosomal veins which receive blood from the meso- 

 soma, (6) visceral veins which conduct the blood from the liver, stomach, and intestines, 

 etc., (7) afferent branchial veins and efferent branchial veins which carry blood to and 

 from the gills, (8) a pair of afferent longitudinal veins which are closely associated with 

 the kidney tissue and receive blood from the veins of the mesosoma and the branchial 

 vessels from the horizontal veins by way of the plicate canals, and from the horizontal 

 vein, visceral veins, intermuscular sinus, and branchial veins through the kidney, (9) a 

 pair of anastomosing veins and the transverse sinus of the posterior adductor muscle 

 which unite the horizontal veins with the longitudinal veins, and (10) a pair of afferent 

 oblique veins which receive the blood from the longitudinal vein and conduct it to the 

 heart. 



The marginal sinus follows the free border of the mantle which is enveloped by the 

 fold of periostracum that extends beyond the edge of the shell (figs. 134 and 135, A/5, 



