GASTEROPODA. 



381 



lions of Reaumur that we are indebted for our 

 knowledge concerning this interesting proces*, 

 and subsequent writers ha\c added little to the 

 information derived from his researches; in 

 order, however, to lay before the leader the 

 principal facts connected with this subject, we 

 shall commence with the simplest forms of the 

 process, and gradually advance towards such as 

 are more complicated and less easily under- 

 stood. 



The shells of the Gasteropoda are of two 

 kinds, some being entirely concealed within 

 the substance of the mantle, and consequently 

 internal, whilst others are placed upon the sur- 

 face of the body external to the soft integument. 

 In the former case the shell is uniform in tex- 

 ture and colourless ; in the latter, its develope- 

 ment is much more elaborate, and it is not un- 

 frequently moulded into a great diversity of 

 forms, and painted with various tints, which are 

 sometimes of great brilliancy. The internal or 

 dermic shells are found in many of the pulmo- 

 nary and tectibranchinte orders, and possess 

 but little solidity; although inclosed in the 

 substance of the mantle, they are so little adhe- 

 rent, that when exposed by an incision they 

 readily fall out of the cavity in which they are 

 lodged, and from which they are apparently 

 quite detached. Their substance is generally 

 calcareous, but in many instances, as in Aplysia, 

 the shell is of a homy texture, being transpa- 

 rent, flexible, and elastic, as is the gladius of 

 many of the Cephalopod Mollusca. In all 

 cases horny or calcareous plates of this descrip- 

 tion are found to be composed of superposed 

 lamella;, which are successively secreted by the 

 floor of the cavity in which they are contained, 

 the inferior layer being always the largest and 

 most recent. These shells, therefore, may be con- 

 sidered as merely formed by the deposition of 

 successive coats of varnish, which become indu- 

 rated, and the simple manner of their growth will 

 best exemplify the mode in which more compli- 

 cated shells, whatever be their form, are con- 

 structed. External shells present an endless di- 

 versity of figure, and some classification of their 

 principal forms will facilitate our contemplation 

 of the peculiarity observable in each. The con- 

 cealed .shells, which are merely the rudiments of 

 what we arenowconsidering,are so small in com- 

 parison with the size of the body, that they can 

 only be looked upon as serving for the protec- 

 tion of the more important organs, namely, the 

 heart and respiratory apparatus, which are placed 

 beneath them, but the external shells, from 

 then- ureat developement, are not merely a partial 

 protection to the animal, but in most cases 

 constitute an abode into which the creature 

 can retract its whole body. The external shell 

 consists generally of one piece, the form of 

 which may be symmetrical, in which case it is 

 a cone or disc simply covering the back of the 

 animal ; or, as is generally the case, the shell 

 may be more or less twisted around a central 

 axis, forming a convoluted, turbinated, or spiri- 

 valve shell. In one genus only, Chiton, Lin., 

 the -.hell is formed of several pieces articulated 

 w-it'n each other, and covering the surface of the 

 back. 



The shell of the Patella, a section of which 

 is represented in//^. 17ft, is a snnpV enn.- |.!ac> d 

 upon the l>ack of the creature, which it com- 

 pletely covers, and upon which it is evidently 

 moulded. On making a section of the animal, 

 as in the figure, the shell is found to be entirely 

 lined by the mantle a, b, by which it is secreted. 



Fig. 178. 



That the whole surface of the mantle is capable 

 of secreting the calcifying fluid from which the 

 shell is formed, is distinctly proved by the 

 manner in which a fracture or perforation in 

 any part is speedily repaired by the deposition 

 of a patch of calcareous matter beneath it, but 

 in the ordinary growth of the animal the differ- 

 ent portions of the mantle execute different 

 functions. It is obvious that the enlargement 

 of the body of the patella, as its age increases, 

 must necessitate a corresponding enlargement 

 of its habitation, and this is principally effected 

 by additions of calcareous matter in succes- 

 sively larger rings around the mouth of the 

 shell only ; the great agent therefore in forming 

 the shell is the margin of the mantle, 4, l>. This 

 hangs loosely as a fringe near the mouth of the 

 shell, and being moveable at the will of the 

 animal, the calcareous matter which it pre- 

 eminently furnishes may be laid on in succes- 

 sive layers to extend the mouth of its abode ; 

 and these consecutive additions are indicated 

 externally by concentric lines running parallel 

 with the circumference of the shell, the num- 

 ber of which necessarily increases with age. 

 Whilst the abode of the creature is thus en- 

 larged by the deposition of shell from the vas- 

 cular and spongy margins of the mantle, the 

 office of the rest of that membrane is reduced to 

 the increase of its thickness, depositing succes- 

 sive coatings of calcareous particles, which are 

 laid on to its inner surface, and when a section 

 of the shell is made (fj, these last-formed strata 

 are readily distinguishable by their whiteness 

 and different arrangement. So far the produc- 

 tion of an external shell is entirely similar to 

 what we have met with in the formation of the 

 internal defences of the naked Gasteropoda, yet 

 in other respects the former are much more ela- 

 borately organised. In the first place many of 

 them are adorned externally with colours, not 

 unfrequently arranged with great regularity and 

 beauty ; these tints belong exclusively to the 

 outer layers of the shell, that is, to those formed 

 by the margins of the mantle, and are produced 

 by a glandular structure appropriated to the 

 secretion of the colouring matter, which only 

 exists in the vascular circumference of the cal- 



