CHAPTER I: A TYPICAL UNIVALVE MOLLUSK 



The Pear Conch well exhibits the structure of the univalves. 

 It is abundant from Cape Cod to Florida. On the same general 

 plan all snails are built. 



THE SHELL 



It is a conical tube spirally wound to the right about a central 

 axis, the columella. The closed, pointed end is the apex; the 

 coiled whorls form the spire. The last coil is called the body 

 whorl, for in it the body lies. The spout-like prolongation is the 

 anterior canal. The spiralchannel is the suture. The shell's mouth 

 is called the aperture. The outer lip is opposite the columella, or 

 inner lip. Lines of growth cross the whorls, close together, and 

 parallel to the outer lip. The hairy skin covering the shell when 

 it is alive is the epidermis. Lining the interior is the enamel. 

 Between these is the main shell substance composed chieflyof lime. 



When the body is drawn into the shell the aperture is closed 

 by a horny door, the operculum. 



Hold the spiral shell of the conch by its long stem, the an- 

 terior canal; the apex is pointed upward; the aperture is at the 

 right. The anterior parts of the body reach the aperture; the 

 posterior parts extend towards the apex. The ventral part of 

 the body is underneath when the foot is extended. It is the 

 inner, short side of the coil. The dorsal part is the arch of the 

 body, the long, outer side of the coil. These terms are necessary 

 to designate the relative positions of the internal organs. 



THE SOFT PARTS 



The mantle is the fleshy web that covers the body and 

 lines the shell. In the spire it is attached to the body, and is 

 very thin. In the body whorl it is free from the body, and en- 

 closes the mantle cavity. It forms a thickened collar around the 

 body, and fits the shell aperture. This collar is attached at 



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