Phylum 7. MolluSCa. 



Tlie body is unsegmentedj very varied in form, and witliout 

 jointed appendages. The skin is soft, often ciliated over large tracts; 

 the cuticle absent or (usually) very thin. The body-wall forms, 

 ventrally, a muscular foot, which is either discoidal or com- 

 pressed, and, on account of its great contractility, forms an important 

 locomotor organ. Anteriorly there is a more or less well-developed 

 head with a mouth, often also with tentacles and eyes. Above 

 the foot and head there is a fold of skin, the mantle, which 

 extends round the whole animal ; in some instances it is narrow ; 

 iu others it forms a large lamellar expansion on either side of the 

 body (Lamellibranchs) ; in others again it is better developed either 

 at the anterior or the posterior end than elsewhere (Gastropods, 

 Cephalopods), formiing a pouch, the pallial chamber, between 

 the body and the mantle {k in Fig. 239 and 268 B). In the 

 majority, the greater part of the animal is covered by an open 

 shell, secreted by the skin, with which it is usually only 

 connected at isolated spots, whilst for the most part it lies free 

 upon the upper surface of the body. The shell is never cast (like 

 the cuticle of Arthropoda), but is continually increased in size by 

 the formation of new material at the edge, whilst it is thickened by 

 deposits from within ; it consists of a substance, conchioliu, 

 something like chitin, but differing from it chemically, and usually so 

 thoroughly impregnated with calcareous salts, principally 

 carbonate of lime, that these constitute the chief part of the 

 shell. 



The alimentary canal usually exhibits a large expansion,, 

 the stomach; the anus is either at the hind end of the animal or is. 

 moved to one side. Salivary glands opening into the mouth 

 are generally, and a well-developed liver invariably, present. In 

 the majority of the Mollusca (with the exception of all Lamellibranchs) 

 there is, on the floor of the mouth a muscular pad, the tongue, 

 covered on its upper surface with a thin, stiff membrane, the- 

 r a d u 1 a , or lingual ribbon, on which are arranged transverse rows- 



