DIGESTIVE ORGANS. 25 



proboscis commences at its extremity (like that of the tentacles 

 of snails) by means of two retractor mnscles attached to the oral 

 mass. ' The same observer describes a muscular disk on the 

 under side of the proboscis, behind the mouth, in Natica and 

 Sigaretus. This disk possesses suctorial action and probably 

 enables the mollusk to attach itself firmly to the shells of other 

 species when drilling them for the purpose of devouring the soft 

 parts. 



The oral mass is usually an oval body formed by invagination 

 of the external skin through the mouth, from the upper posterior 

 end of which proceeds the <x?sophagus, whilst at the lower pos- 

 terior end is situated the lingual sheath, enclosing the odonto- 

 phore. The wall of the oral cavity is filled mostly with flesli- 

 colored muscles and clothed with an epithelium, which is often 

 covered b}' a thick cuticula, and furnished with cilia on the roof 

 of the mouth. 



The lips form a short hollow cylinder at the commencement of 

 the mouth, made up of longitudinal and annular muscles, the 

 latter preponderating ; and sometimes forming a ring-like thick- 

 ening, which is greatly developed and cleft into lobes in the 

 genus Conus. and forms another kind of sucking disk, by the use 

 of which the animal assists its locomotion necessarily laborious 

 on account of the weight of its shell. In the terrestrial branch- 

 iferous genus Cyclostoma the snout possesses a similar disk. 



Jaws. The inner surface of the lips is sometimes covered by 

 hard plates, which are evidently of much service in grasping and 

 comminuting food. The jaws are attached by their hinder por- 

 tions to the labial skin or membrane, from the epithelium of which 

 they are secreted. They are hyaline, without structure, and yel- 

 lowish. Their front face is detached from the membrane and 

 frequently elevated like a scale, bearing sometimes, as in Dolium 

 galea, a rounded free hook. In the Prosobranchiates the jaws 

 are a pair, situated on either side, but in some of the Pulmonates 

 this pair become united above, forming a single, arched, superior 

 jaw. These cheek plates or immovable mandibles are found in 

 nearly all the Taenioglossata, as well in those provided with a 

 rostrum (Cyclostoma, Valvata, Rissoa, Jeffrey sia, Crepidula, 

 Vermetus, Trichotropis, etc.) as in those with a haustellum 

 (Marsenia, Xatica, Cyprsea, Cassis, Triton, Strombus, etc.). 

 4 



