BRITISH MOLLUSKS. 



Class I. CEPHALA — with head. 



The two Classes of Cephala and Acephala, signifying Headed and 

 Headless, into which Land and Freshwater, and indeed all mol- 

 lusks, are primarily divided, correspond as near as may be with the 

 old Linnsean designations of Univalve and Bivalve. The Acephala, 

 which are all water-dwelling mollusks, are always provided with a 

 shell; but the Cephala, or Gastropods, as they are also called, are 

 sometimes, as in Avion, a garden slug, without any shell ; the term 

 Univalve is then inappropriate. There exist about a hundred and 

 twenty species of land and freshwater mollusks in Britain, fifteen of 

 which — the Mussels and Cyclads — are acephalous, without a head. 

 The remaining hundred and five, representing twenty -nine genera, 

 belong to the Class under consideration. They possess a well-deve- 

 loped head, with either two or four tentacles, having a pair of eyes, 

 sometimes at their extremity, sometimes at the base ; and the mouth 

 is provided with a jaw and minute palate-teeth which present some 

 curious varieties of structure. The body of the Cephals is elongated 

 and rather depressed, the upper surface developing a mantle which 

 is the calcifying organ, secreting, in most instances, a shell covering 

 or enclosing the respiratory and visceral organs of the animal. The 

 lower surface of the body forms a grasping fleshy disk, by the con- 

 traction and dilatation of which the creature crawls. 



Tribe I. INOPERCULATA — without operculum. 



Attention may now be directed to the Synoptical Table opposite. 

 The class Cephala, it will be seen, is subdivided into Inoperculated 

 and Operculated. The Inoperculated Cephals, those unprovided 

 with any operculum, comprise the pulmoniferous or air-breathing 

 families, Limacinea, Colimacea, Auriculacea, including about half 

 of the entire series ; and the pulmobranchiate or lung-gilled family 

 Lymnceacea breathing both air and water. The Operculated Cephals 

 comprise the pulmoniferous family Cyclostomacea, breathing air only, 

 and the branchiferous or gilled families Peristomata and Neritacea, 

 breathing water only. There is, however, no absolute line of de- 

 marcation between them. All are modified and run into each other 

 by gradations of character. The Ampullarice, a foreign genus of 

 Peristomata, though breathing water as their natural element of 



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