MOLLUSCA. 125 



The shell consists of three layers. There is a thin external 

 chitinous periostracum, which is pigmented, and gives colour to 

 the shell. Below this is a dense prismatic layer, and internally a 

 thinner pearly layer, composed of numerous lamina?, and with a 

 smooth and polished internal surface. The last layer can be 

 secreted by all parts of the epidermis covering the visceral hump, 

 but the two others can only be formed by the collar, which, as 

 the animal grows, adds successive increments to the mouth of 

 the shell, the boundaries between which are indicated by lines of 

 growth. 



3. The digestive organs (Figs. 35, 36) consist of a convoluted 

 gut running from the anterior ventral mouth to the unsymmetri- 

 cally placed anus, and receiving the secretions of salivary and 

 digestive glands. The gut is divisible into buccal mass (pharynx), 

 gullet, crop, stomach, intestine, and rectum. 



The mouth leads into a large mouth-cavity, contained in an 

 oval muscular buccal mass. Immediately within the lips, dorsally, 

 is a crescentic, horny, toothed structure, the jaw. But the most 

 important organ connected with the mouth is the odontophore. 

 This consists of an elevation rising up like a tongue from the 

 floor of the mouth, on which .a horny ribbon (the radula), bearing 

 innumerable minute pointed teeth, is spread out from back to- 

 front, passing behind into a pouch, the radular sac, which lies 

 at the back of the buccal mass, and forms a small rounded pro- 

 tuberance of whitish colour. Into the outside of the buccal mass 

 special muscles are inserted, the buccal protractors and retractors. 

 From the upper side of the buccal mass a narrow, thin-walled 

 gullet (oesophagus) passes back, and merges into a spindle- 

 shaped crop, the thin walls of which are marked by longi- 

 tudinal striations. The crop narrows behind, and is succeeded 

 by a rounded stomach with moderately thick walls. This turns 

 sharply upon itself, and is then followed by the narrow, thin- 

 walled intestine, which first bends ventrally, and then, after 

 coiling a little, passes into the rectum, which takes a straight 

 course along the right side of the lung-chamber to open by 

 the anus. Projecting into the intestinal cavitv is a longitudinal 

 fold. 



The salivary glands are paired branching structures, placed one 

 on each side of the crop, to which they are attached by connective^ 

 tissue. A slender salivary duct runs from each of them down the 



