PTEROPODA AND GASTROPODA. 545 



a shell during the lifetime of the inhabitant. This occurs to the 

 apex of certain univalves after the shell has been evacuated by the 

 original occupant in the widening and lengthening the shell to 

 accommodate it to an increase of bulk : such shells are said to be 

 " decollated," as, for example, the Bulinus decollatus. 



The inhabitants of univalve shells dispose in different ways of that 

 part of their calcareous abode which they evacuate in the progress of 

 growth ; in the decollated shells the vacated spire is portioned off, 

 prior to its abrasion, by the formation of a thin nacreous plate. In 

 the Vermetus gigas the vacated portions of the tube are retained, and 

 successively portioned off, a series of concave plates or septa being 

 thus developed. In the Magilus antiquus the posterior part of 

 the shell, as the soft parts move forwards, is progressively tilled up 

 with a dense, solid, subtransparent, crystalline deposit of carbonate 

 of lime. 



Univalves which inhabit rocks upon which surf-waves are ever 

 breaking, have stronger and denser shells than those that live in 

 calmer seas, or in sandy and muddy bottoms. The shells of air- 

 breathing mollusks exhibit analogous effects of external influences ; 

 those of the burrowing Bulini, e. g., are colourless and subtrans- 

 parent ; but the shell is vividly coloured in the species inhabiting 

 plains with scanty vegetation and much exposed to solar light : the 

 shells are largest and thickest in the arboreal Bulini of tropical 

 forests, which live amongst an abundance of decaying vegetable 

 matter. 



The part of the mantle which invests the viscera in the conchifer- 

 ous Gastropods is smooth, thin, and sub-transparent, resembling the 

 sac of a hernia, which, with the viscera themselves, appears to have 

 escaped from the common muscular integument of the body. This 

 visceral mass, as it is termed, is lodged in the upper part of the cone 

 of the shell, the spiral turns of which it follows. The head and foot 

 of the animal can be protruded from the mouth of the shell, and be 

 retracted within its last whorl, by the action of a muscle, which has 

 its fixed point in the columella of the shell. This retractor, to which 

 the operculum when present is attached, answers to the posterior 

 retractor of the foot in bivalves. The form and size of the shell- 

 aperture correspond with, and indicate the size of, the foot. In the 

 pectinibranchiate Mollusks, which are the chief fabricators of the 

 beautiful turbinated shells of the conch ological cabinet, the foot is 

 attached to the anterior part of the body by a narrow base ; whence 

 they have been termed by Lamarck Trachelipods. 



The primitive muscular fibre is smooth in all Gastropods : the pri- 

 mitive fasciculi have often numerous nuclei scattered through them. 



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