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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. 183. Whelk (Buccinuni). 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 (Valuta), Olive- 

 Shells (Olivd) (fig. 184), Harp- 

 Shells (Harpa), Cone -Shells 

 (Conus\ Turret-Shells (Turritella\ Wing-Shells (Strombus) 

 (fig. 185), Helmet-Shells (Cassis), and Cowries (Cypraa) (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 (Pterotr ached) is 

 entirely absent. 



Fig. 184. Olive (OKva) 



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