173 



Describe fhe General Structure of Anodonta. 



The main portion of the body is between the upper parts 

 of the shell ; a keel-like extension is the visceral mass, 

 which contains the middle portion of the gut, the " liver," 

 and the reproductive organ. This mass is continued down- 

 wards and forwards as the muscular and median wedge- 

 shaped protrusible " foot." 



Two large gill-plates, attached along each side of the 

 body, hang down at each side of the " foot." The gills 

 and "foot" are within the mantle-cavity, the large space 

 enclosed by the two mantle lobes or flaps (downgrowths 

 of the body wall) which line the shell. At the posterior 

 end of the shell, the mantle lobes form two siphon-like 

 openings of the two chambers into which the mantle-cavity 

 is divided by a partition at the bases of the gills. Water 

 enters by the large ventral inha4ent opening into the large 

 infra-branchial chamber, and passes through the gill- 

 plates (between the filaments) into the small upper or 

 supra-branchial chamber, and along that chamber it flows 

 backwards to the small dorsal exhalent opening. The 

 current is kept going by the cilia on the mantle and gills. 



Two large muscles, anterior and posterior adductor, 

 which stretch across from valve to valve, close the shell ; 

 and beside these are the smaller retractor and protractor 

 muscles of the "foot." 



. In front of the gills are the labial palps, two pairs of 

 small triangular flaps forming the lips of the mouth which 

 is below the anterior adductor muscle. The heart, which 

 consists of two auricles and a median ventricle, lies within 

 a pericardium situated beneath the dorsal hinge-ligament. 

 The ventricle surrounds the rectum, the terminal portion 

 of the gut, which, passing upwards from the visceral mass 

 traverses the pericardium and opens (anus) into the supra- 

 branchial chamber near the dorsal exhalent opening. 

 The paired kidney (Organ of Bojanus) lies below the peri- 

 cardium, and communicates with it through two crescentic 

 reno-pericardial openings (nephrostomes) at the anterior 

 end of the pericardium. The ureter opening is where the 

 inner gill -plate is attached to the visceral mass ; and just 

 below it is the genital aperture. 



