A Tjrpical Univalve Mollusk 



certain points in tiie margin, folded and prolonged at others. 

 The large opening permits the foot to protrude. 



The siphon is a tubular prolongation of the mantle, which 

 fits into the anterior canal. The muscular organ that bears the 

 operculum is the foot. The pedal gland opens on the sole, or 

 disc of the foot. An anterior fold of the mantle border forms the 

 head. A central, retractile prolongation, the proboscis, contains 

 the radula — odontophore or rasping tongue— a flexible band set 

 with many transverse rows of teeth, which may be seen protruding 

 from the mouth, at the tip. The central rachidian tcoth in each 

 row is flanked by the laterals; on the borders are the marginals 

 or uncini. On each side of the head is an erect trian- 

 gular projection, the tentacle; each bears a dark eye on the 

 outer edge. 



The rounded arch of the body is the visceral dome. The 

 mantle, transparent and thin here, forms the wall of the body. 

 Through it various internal organs are visible. In the first two 

 coils from the apex is a dark mass, the liver. The reproductive 

 gland, brown, red or yellow, overlies the dorsal surface of the 

 liver. The stomach, curved, light-coloured, often indistinctly 

 seen, is just under the surface and overlies the liver on the left. 

 The kidney, somewhat rectangular, yellowish brown to chocolate- 

 coloured, lies on the left side, anterior to the reproductive glands. 

 The yellowish, two-chambered heart lies in a triangular sac, the 

 pericardium, anterior to the kidney. 



In females a large yellow nidamental gland lies over the back 

 of the visceral dome, and along the side of the columellar muscle, 

 which fastens the body to the shell. In front of the heart, and 

 extending its overlapping plates into the mantle cavity, is the 

 large, brown gill. The osphradium is a small brownish organ to 

 the left of the anterior end of the gill. To the right of the gill 

 is the hypo-branchial gland. 



Slitting open the mantle straight backward along the right 

 side of the gill, and turning back the flaps, the mantle cavity 

 lies open. Here on the right side is the anus, the opening of 

 the intestine, on a short papilla. The opening of the nidamental 

 gland is near by on another papilla, to the right and in front of 

 the anus. The corresponding gland in the male is the testis. 

 The large external male organ which resembles the proboscis, 

 but is shorter, rises on the head behind the right tentacle. The 



23 



