GASTEROPODA. 297 



iiised, is well illustrated by the thinning of the parietes of the internal 

 whorls of the Cones and Olives, from which two out of the three layers 

 of which they were originally composed may be observed to have 

 been removed. The absorption of shell is also illustrated by the re- 

 moval or smoothing down of the spines of the 3Iurices ; by the flat- 

 tening of the inner lip of the mouth of the Purpurce ; by the widen- 

 ing of the foecal aperture of the Fissurellce ; and it gives rise to 

 various other modifications in the form and structure of shell in the 

 progress of growth. Another change of form is due to the physical 

 decomposition or destruction of a part of a shell during the lifetime 

 of the inhabitant. This occurs to the apex of ceriain Univalves after 

 the shell has been evacuated by the original occupant in the widen- 

 ing and lengthening the shell to accommodate it to an increase of 

 bulk. Such shells are said to be " decollated," as, for example, the 

 Helix decollata. 



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 ; a similar structure is indicated in the remains of the 

 large elongated fossil shells, called Icthyosarcolites. In the Magilus 

 antiquus the posterior part of the shell, as the soft parts move for- 

 wards, is progressively filled up with a dense, solid, subtransparent, 

 crystalline deposit of carbonate of lime. 



The part of the mantle which invests the viscera in the conchiferous 

 Gasteropods 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. The form and size of 

 this compartment of the shell, and of its aperture, correspond with, 

 and indicate the size of, the foot. 



In the Pectinibranchiate Mollusca, which are the chief fabricators 

 of the beautiful turbinated shells of the conchological cabinet, the 

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

 whence they have been termed by Lamarck Trachelipods. 



These with the true Gasteropods are organised to subsist on a great 

 variety of food : they select both animal and vegetable matter in both 



