VENUS MERCENARIA LINN/EUS 209 



growth form around each umbo and appear in the adult shell as 

 a distinct prominence or shoulder. The valves are held together 

 by a ligament and a lock. The latter is one of the few structures 

 which are not strictly symmetrical in Venus, for the teeth of one 

 shell correspond to notches in the other. The teeth are of two 

 kinds. In front of the ligament are prominent Cardinal teeth, 



FIG. 41. Anodonta cygnca. General Anatomy, from Parker and Has- 

 well's Text-book of Zoology, a, anus; a. ad, anterior adductor; a. ao, an- 

 terior aorta; a. v. ap, auriculo-ventricular aperture; bl, urinary bladder; c. 

 pl. gn, cerebro-pleural ganglion; d. d, duct of digestive gland; d. gl, diges- 

 tive gland; d. p. a, dorsal pallial aperture; ex. sph, exhalant siphon; //, foot; 

 g. ap, genital aperture; gon, gonad; gul, gullet; i. l.j, inter-lamellar junction; 

 in. sph, inhalant siphon; int, intestine; kd, kidney; m, mantle; mih, mouth; 

 p. ao, posterior aorta; p. ad, posterior adductor; p. c, pericardium; pd. gn, 

 pedal ganglion; r. ap, renal aperture; r. an, right auricle; ret, rectum; 

 r. p. s, reno-pericardial aperture; st, stomach; ty, typhlosole; v, ventricle; 

 v. gn, visceral ganglion; w. t, water tubes. 



while under the ligament are the smaller and more numerous 

 lateral teeth. The free edge of the shell is serrated, this being 

 due to its origin as a secretion of the fringed edge of the mantle 

 and allowing a firmer closing of the two valves. The shell is 

 composed of three layers: the outer layer or periostracum, the 

 middle or prismatic layer, and the inner or mother-of-pearl layer. 



