(fig. 81). They have a distinct layer of circular 

 and longitudinal muscles surrounded by connec- 

 tive tissue. 



The arterial system described liere is shown 

 diagrammaticaUy in fig. 232 from the right side, 

 after the partial removal of the mantle and some 

 of the visceral mass. The right wall of tlie peri- 

 cardium is cut off to expose the heart. This 

 diagrammatic drawing is based on examination 

 of several specimens injected through the ventricle. 



Two large arteries emerge from the posterio- 

 dorsal side of the ventricle. Tlie largest one is the 

 anterior aorta (ant.ao.), which upon leaving the 

 lieart forms a short enlargement or a bulb leading 

 to the large visceral arteiy (vise. a.) with its nu- 

 merous branches and small pericardial arteiy 

 (small unmarked vessel under the visceral artery), 

 which supplies blood to the wall of the pericardium. 

 The much smaller posterior aorta (post.ao.) sup- 

 plies blood to the adductor muscle and rectum (r.). 

 Near the point of emergence of the posterior aorta 

 it gives off a small rectal artery (r.a.), which fol- 

 lows the wall of the rectum. 



The visceral artery (vise. a.) emerges from the 

 anterior aorta as a wide vessel that supplies blood 

 to the organs of the visceral mass. Its upper 

 branch reaches the level of the lalnal palps and of 

 the cephalic hood. The lower l>ranch extends 

 along the wall of the crystalline sac and forms the 

 reno-gonadial artery (r.g.a.) ; numerous small 

 branches of this vessel supply blood to gonads and 

 kidneys. 



In its course toward the anterior part of the 

 body, the anterior aorta (ant.ao.) passes under the 

 intestinal loop (not shown in fig. 232) and gives off 

 several small vessels which bring blood to the 

 digestive diverticula (gastric arteries, g.a.), man- 

 tle, and the labial palps. At the anterior end of 

 the body the aorta forms a common trunk of the 

 pallial artery (co.p.a.), which divides into two 

 short branches corresponding to the left and right 

 side of the body, each branch giving rise to the 

 ventral and dorsal circumpallial arteries (cr.p.a.). 

 Each of these continues along the peripheiy of the 

 mantle lobes, supplying blood to the mantle 

 through a large number of sliort vessels which entl 

 in the mantle lacunae. A very small subliga- 

 mental artery emerges from the end of the common 

 pallial arter\- and leads to the subligamental glanfl 

 (fig. 78). Tlie cephalic artery- (cph.a.) and labia! 

 arteiy (l.a.) supply blood to the anterior end of the 

 body and to the right and left labial palps. 



254 



THE VENOUS SYSTEM 



Since the presence of irregular sinuses prevents 

 the filling up of the entire venous system with one 

 injection it is necessary^ to make separate injec- 

 tions of the principal vessels and to supplement 

 the study with an examination of sectioned mate- 

 rial. The course of small veins may be traced by 

 injecting a water soluble dye and watching its 

 penetration in tlie tissues of the visceral mass and 

 gills. 



The venous system comprises the sinuses, af- 

 ferent and eft'erent veins and small vessels of the 

 gills. It is diagrammaticaUy sliown in figure 233. 

 Ramifications of the vessels are omitted for the 

 sake of clarity. 



The sinuses occur throughout the entire visceral 

 mass, in the pallium, along the adductor muscle, 

 and around the kidneys. Their outlines are 

 highly irregular, and tlie area thej' occupy varies, 

 depending on the degree of distension by blood. 

 Tlie renal sinus (r.s.) consists of several smaller 

 sinuses which surround the main part of the 

 kidneys and open into the efferent branchial 

 vessel and into the sinuses between the adductor 

 muscle and the heart at the posterior side of the 

 body. The renal sinus spreads into the connec- 

 tive tissue of tlie adjacent area and is in communi- 

 cation with the inter-nephridial passages leading 

 to the pericardium. The renal vein (r.v.) carries 

 blood from the sinus into the common afferent 

 vein. The visceral sinus, v.s., not definitely 

 outlined in the diagram, spreads over the surround- 

 ing tissues and drains its blood through the 

 gastric (g.v.), hepatic (h.v.), and other veins into 

 the common afferent vein (c.af.v.). The muscle 

 sinus (m.s.) is a small area below the renal sinus 

 on the surface of the adductor muscle under the 

 pyloric region. The system of afferent veins 

 consists of a single common afferent vein (c.af.v.) 

 and two lateral afferent veins, 1. af. v. (fig. 233 and 

 fig. 73). The common afferent vein runs on the 

 ridge formed by the fusion of the two inner 

 ascending lamellae of the gills. The blood 

 received by this vein conies from tlic deeper 

 parts of the Ixidy and is l)rought by a number of 

 veins which can be identified as the ceplialic 

 veins (c.v.) from the cephalic region; tlie labial 

 veins (l.v.) ; the gastric and hepatic veins (g.v., 

 h.v.); the network of small reno-gonadial veins 

 (r.g.v.); short renal vein (r.v.) and the adductor 

 muscle vein (not shown in the diagram). In 

 thin, wateiy specimens most of these veins can 1 



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