116 CONCHOLOGICAL NOTES. 



The Whelk is a typical illustration of the Class 

 GASTROPODA, and also of the Conchological Division, 

 Univalve Shells. 



The PROBOSCIS, which is evaginable from the base, is 

 the organ through which the food is conveyed to the 

 stomach. At its extremity is situated the tooth -ribbon 

 (TOOTHED BAND), or radula. This organ consists of a 

 ribbon of chitinous or horny membrane, upon which are 

 situated siliceous teeth of various shapes and sizes. The 

 teeth are arranged in transverse rows, and as the front 

 part of the radula is worn away, new teeth from behind 

 are brought into use.* They are arranged in definite 

 order : that in the median line of the radula is known as 

 the rachidian, on either side of this, the laterals. The 

 central tooth has seven cusps, the laterals, four cusps each, 

 the marginal cusp of each lateral tooth is greatly extended 

 and curves towards the median line. The radula bears 

 100 rows of teeth. The TENTACLES are two in number, 

 each having an eye situated at the base. The EYES 

 consist of cornea, lenses, retina, and optic nerve. 



The SYPHON is a spout-like continuation of the mantle 

 edge. It conducts water into the mantle-cavity. 



The MANTLE completely enfolds the body at the 

 junction of visceral sack with the head and foot. In one 

 part of the mantle is a deep groove which forms the 

 mantle-cavity, within which are the ctenidium, anus, renal 

 orifice, and generative opening. On the surface of the 

 mantle are the glands, which secrete the shell-producing 

 materials and the periostracum. 



The FOOT is a long, and somewhat broad, muscular 

 creeping disk, projecting from the ventral surface of the 

 body. Its action impels the creature in a steady gliding 

 manner. 



GILL or ctenidium. The whelk possesses but one 

 ctenidium. It is attached for the greater part of its 

 length to the mantle-wall, and bears a row of plates 

 arranged perpendicularly to the axis. 



The MUSCLE OF ATTACHMENT is affixed to the body at 

 one end and to the interior of the shell at the other end. 

 By its elongation the animal is exerted from the shell, and 

 retracted by its contraction. 



REPRODUCTORY ORGAN. The whelk is dioecious, and 

 there is little external indication of the sexes. 



The whelk has but three teeth. 



