THE SKIN. 7 



culum (when the latter is developed, which is not always the 

 case . Although there is only this single actual bond of con- 

 nection between shell and animal, the contact of the body serves 

 to maintain the vitality of the shell, which soon bleaches, and 

 final ly decays when separated from its architect and inhabitant. 

 We will now describe, in a necessarily cursory manner, 

 successively, the skin, the foot, the muscles, the shell and 

 operculum, the digestive, nervous, vascular, respiratory, secre- 

 tive and sexual organs of the prosobranchiates. 



The Skin. 



The body is completely enveloped by an external skin. Its 

 epithelial layer is formed of quadrangular or prismatic cells, 

 which have a distinct nucleus, and occasionally, when long- 

 cylindrical in form, they have a tail-like end beneath, penetrating 

 the cutis.* Externally the epithelium sometimes supports cilia 

 upon the exposed portions of the body. 



The cutis consists essentially offline interlacing muscular fibres 

 with interposed cells ; often attaining a perceptible thickness. 

 The subcutaneous muscular layers of the body are immediately 

 continuous with those of the cutis : the fibres of which they are 

 formed may be clearly distinguished as an outer longitudinal 

 and inner circular layer. These fibres are (as in inollusks gene- 

 rally) compressed and band-like, with pointed ends and central 

 oval nucleus. 



The skin forms a fold above and surrounding the foot, and 

 this portion is technically termed the mantle ; it surrounds the 

 bod}', behind the cephalic portion like a collar, and thence spreads 

 dorsally over the posterior part of the animal. In the terminal, 

 thickened border of the mantle, the cutis becomes of greater 

 thickness ; its upper stratum containing very numerous glands, 

 furnishing the mucus and colors which are here mingled with the 

 secreted shell-material. Similar glands, furnishing a copious 

 supply of mucus are found also in the cephalic portion of the 

 body as well as in the foot, and especially in the sole ; but no 

 cells are found in that part of the animal permanently covered 



* It is possible that these last are epithelial terminations of nerves 

 hence sense organs. 



