SEA MUSSEL MYTILUS EDULIS. 157 



long actively beating cilia which keep up a constant flow of water. Sabatier (1874) was 

 the first to recognize these as respiratory organs and noted how important it was for 

 them to take on this role when the mantle was distended with genital products. This 

 view is in harmony with the fact that when the mantle is filled with reproductive elements 

 the plicate canals are distended with blood and are most prominent. A small amount 

 of blood from the mantle may flow into the kidney without passing through the plicate 

 canals. 



In the kidney certain impurities are removed from the blood, as has been stated 

 in the account of the excretory system. 



The return of the blood to the heart from the kidney may be by two channels, either 

 directly by way of the longitudinal vein and the oblique vein, or by way of the branchial 

 vessels, anterior longitudinal vein, and the oblique vein. By far the greater part of the 

 blood takes the first-mentioned course. The branchial circulation of Mytilus is very 

 weak compared with that of many other lamellibranchs. Sabatier (1874) assigned 

 three reasons to account for this: (1) The small caliber of the branchial vessels, (2) the 

 feeble course of the blood which comes to the gills after traveling an extensive capillary 

 network, and (3) the existence of more easy paths of return which allow the blood to 

 reach the heart without traversing the gills. 



A diagram of the general course taken by the blood in Mytilus is shown in figure 138. 



MUSCULAR SYSTEM. 



The muscles of Mytilus e Jul is fall naturally into five groups: (1) The adductors 

 which close the valves, (2) the muscles of the foot, (3) the retractor muscles of the foot 

 and byssus, (4) the pallial muscles which attach the mantle to the border of the shell, 

 and (5) the anal muscles. 



The adductor muscles are two in number, consisting of a small anterior adductor and 

 a large, much more strongly developed, posterior adductor. The ratio in volume 

 between the two muscles varies from 1:8 to 1:10. The posterior adductor (fig. 117, 

 PAd, opp. p. 138; fig. 141, PAd, opp. p. 158) serves as the powerful muscle to close the 

 valves and is located in the posterior dorsal region of the body where it runs across from 

 one valve to the other. Its ends are firmly embedded in the round impressions of the 

 shell which have already been described. The muscle itself is more or less cylindrical 

 in form and is composed of numerous bundles of fibers which run parallel with each other 

 across the space between the valves. Owing to the convexity of the shell the fiber 

 bundles of the lower portion are about twice as long as those on the dorsal surface. 



The anterior adductor (fig. 141, A Ad) lies at the anterior end of the ventral edge of 

 the shell. It extends across from one valve to the other as a thin band of fibers which 

 is traversed on its midventral surface by a narrow pigmented membrane that arises 

 from the union of the inner folds of the right and left mantle edges. The muscle termi- 

 nates on the anterior ventral surface just inside the edge of each valve in the impression 

 shown in figure 104, AAd (opp. p. 132). 



The muscles of the foot are of two types, an outer circular layer of fine fibrils and 



an inner longitudinal layer composed of large bundles of fibers (fig. 145, CM and LM, 



opp. p. 159). The longitudinal muscles make up the bulk of the foot and run its entire 



length. They occupy chiefly the dorsal and lateral portions; in the ventral region they 



90392°— 22 3 



