M U SC U L A R S V S T K M . 



teropods, the alar fibres of the foot extend upwards 



along th( a strong fasciculus is prolonged back- 



wards UIK to be attached to the columella of the 



shell. This tractor muscle forms the only bond of con- 

 nection between the shell and the animal it contains, and it is 

 constantly advancing slowly along the pillar of the shell dur- 

 ing growth like the adductor muscles of conchifera. The open 

 mantle which secretes the calcareous matter of the shell is 

 also distinctly muscular and contractile, and the funnel for 

 respiration is an open canal prolonged from its left side. 

 The foot is often expanded by the introduction of water into 

 its interior cavity, and its dorsal surface secretes the calca- 

 reous or horny layers which compose the operculum of the 

 shell. The neck forms generally a thick muscular sheath 

 around the complex apparatus of the mouth and proboscis, 

 and supports on the right side the muscular exciting organ of 

 the male ; anteriorly it forms the lips or the sheath of the pro- 

 boscis, and the tentacula which have generally the eyes at the 

 exterior of their base. In the naked gasteropods found generally 

 adhering to floating plants in the ocean, as the scyll&a and 

 tritonia, the foot forms a long narrow groved organ for em- 

 bracing the tender stems to which they adhere, and on which 

 they feed. Most of the predaceous gasteropods possess a 

 long and powerful muscular proboscis capable of being ex- 

 tended to a distance from the mouth, and provided at its ex- 

 tremity with an exsertile bifid fleshy tongue armed with 

 sharp conical recurved teeth, as seen in that of the common 



FIG 66. 



whelk, buccinum undatum (Fig. 66.) 

 Both the proboscis (g, g.) and the en- 

 closed bilabiate spiny tongue (a, b.) are 

 provided with numerous powerful ex- 

 tensor and retractor muscles contained 

 in the neck. The two divisions (a.) of 

 the tongue to which the teeth are at- 

 tached are often supported internally by 

 two long cartilaginous laminae, large and 

 strong in the buccinum, which appear 

 to be the analogues of the gastric dart 

 of conchifera. The phytophagous gas- 

 teropods have generally jaws for com- 

 pressing their vegetable food, or a long spiny tongue for 

 filing it to pieces. 





