94 DIGESTIVE ORGANS. 



protrusion of the proboscis, on the contrar^^is efi'ected by press- 

 ing forward tlie blood towards the head, an operation assisted 

 by the contraction of the annular muscles of the fore-part of the 

 body. In Natica, according to Troschel,the invagination of the 

 proboscis commences at its extremity (like that of the tentacles 

 of snails ) by means of two retractor muscles 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 firml}^ 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 oesophagus, whilst at the lower posterior 

 end is situated the lingual sheath, enclosing the odontophore. The 

 wall of the oral cavity is filled mostly with flesh-colored muscles 

 and clothed with an epithelium, which is often covered by 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 evidentl}^ 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 

 yellowish. 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 becomes united above, forming a single, arched, superior 

 jaw (xiii, 58, 60).* These cheek plates or immovable mandibles 

 are found in nearl}^ all the tsenioglossata, as well in those provided 

 wdth a rostrum (C3^clostoma, Yalvata, Rissoa, Jeffre3^sia, Crepi- 

 dula, Yermetus, Trichotropis, etc.) as in those with a haustellum 

 ( Marsenia, Natica, Cyprsea, Cassis, Triton, Strombus, etc.) The}'' 

 are apparently wanting in all the rachigiossata (Murex, Fusus, 

 Nassa, etc.). The linear horny plates described in Buccinum 



* The jaw and lingual ribbon of a large Helix, such as H. albolabris or 

 thyroidea may be i-eadily observed by offering tempting food, such as a 

 piece of lettuce or cabbage, to the animal. 



