SNAILS AND SLUGS 



321 



the mouth drawn out into a long canal for the reception of the 

 siphon. Tyrian purple was obtained from species of Murex 

 and Purpura, the organ yielding it being a gland in the roof of 



Fig. 184.— Olive [Oli-va) 



Fig. 183. — Whelk {Buccinu»i). a a. Tentacles; b, siphon; c, operculum 



the mantle-cavity, of which the juice turns purple on exposure 

 to sunlight. 



The Common Whelk {Buccinum undatum) (fig. 183), inhabit- 

 ing both shallow and deep water around our coasts, closely re- 

 sembles the Purple in structure, 

 but is very much larger. 



As examples of other families 

 may be mentioned: — Mitre-Shells 

 {Mitra), Volutes {Voluta), Olive- 

 Shells {piiva) (fig. 184), Harp- 

 Shells {Harpa), Cone -Shells 

 (Conus), Turret-Shells {Turritella), Wing-Shells {Strombus) 

 (fig. 185), Helmet-Shells {Cassis), and Cowries {Cyprcsd) (fig. 186). 

 In many of these the shells are extremely handsome, and occupy 

 a prominent place in museums and private collections. 



Special mention must be made of the Heteropods, a group 

 of comb-gilled snails which swim freely in the open sea. The 

 body in these pelagic forms is transparent, and the foot is a 

 laterally-flattened fin-like structure, by means of which the animal 

 swims back downwards. The shell may be spiral {Atlanta) or 

 cap-shaped {Carinaria), but in some cases {Pterotrachea) is 

 entirely absent. 



Vol. I. 21 



