13 



becomes very strongly developed. From it a cuticular 

 structureless membrane passes off over the free edge of the 

 shell forming the epicuticula or periostracum (Epic). 

 When the shell is dissolved off in acid, this cuticular 

 investment is seen to be really continuous with the mantle 

 edge, so that the latter is firmly attached to the outer 

 surface of the shell. 



On the inner surface is a ciliated zone which begins a 

 little way back from the free edge of the inner fold, and 

 ceases or begins to die out opposite to the line of attach- 

 ment of the pallial muscles to the shell. Coincident with 

 the distribution of the cilia is that of a zone of mucous 

 glands opening on to this ciliated surface on the mantle 

 margin. These glands are very similar to those which 

 will be described as present on the ciliated tip of the 

 foot (p. 17), but are, as a rule, unicellular, the body and 

 conducting stalk being formed from a single cell. Occa- 

 sionally several cohere together forming a structure of the 

 same nature as those found on the foot, but they are very 

 generally much smaller individually. Their stalks passing 

 out through the epidermis give the latter an irregular 

 appearance. The tips of these stalks project out among 

 the cilia as prominent little knobs. 



The pallial muscles (Rct.m., fig. 23) have a narrow zone 

 of attachment to the shell (Ret.vi'., fig. 10). Here the 

 epidermic cells disappear completely from the mantle 

 surface. The muscle bundles, passing ventrally, parallel 

 to the surface, are attached to the shell at a very acute 

 angle. The bundles run along the outer surface for a 

 short distance, then divide into two series. One of these 

 continues to run along the outer surface, and terminates 

 in the connective tissue in the outer and median folds, 

 the other series, consisting of stronger bundles, crosses over 

 to the inner surface and breaks up into smaller bundles, 



