604 SHELLS OF GASTEROPODS. 



1 Porcellanous ' Shells are composed of three layers, all presenting 

 tbe same kind of structure, but each differing from the others in 

 the mode in which this is disposed. For each layer is made-up of 

 an assemblage of tbin laminae placed side-by-side, which separate 

 one from another, apparently in the planes of rhomboidal cleavage, 

 when the shell is fractured ; and, as was first pointed -out by Mr. 

 Bowerbank, each of these lamina? consists of a series of elongated 

 spicules (considered by him as prismatic cells filled with carbonate 

 of lime) lying side-by-side in close apposition ; and these series are 

 disposed alternately in contrary directions, so as to intersect each 

 other nearly at right angles, though still lying in parallel planes. 

 The direction of the planes is different, however, in the three layers 

 of the Shell, bearing the same relation to each other as have those 

 three sides of a cube which meet each other at the same angle ; 

 and by this arrangement, which is better seen in the fractured edge of 

 the Cyprcea or any similar shell, than in thin sections, the strength 

 of the Shell is greatly augmented. — A similar arrangement, obviously 

 answering the same purpose, has been shown by Mr. Tomes to exist 

 in the Enamel of the Teeth of Rodentia. 



468. The principal departures from this plan of structure are 

 seen in Patella, Chiton, Haliotis, Turbo and its allies, and in the 

 ' Naked ' Gasteropods, many of which last, both Terrestrial and 

 Marine, have some rudiment of a shell. Thus in the common 

 Slug, Liviax rufus, a thin oval plate of Calcareous texture is found 

 imbedded in the shield-like fold of the mantle covering the fore- 

 part of its back ; and if this be examined in an early stage of its 

 growth, it is found to consist of an aggregation of minute calcareous 

 nodules, generally somewhat hexagonal in form, and sometimes 

 quite transparent, whilst in other instances presenting an appear- 

 ance closely resembling that delineated in Fig. 309. — In the Epi- 

 dermis of the Mantle of some species of Doris, on the other hand, 

 we find long Calcareous Spicules, generally lying in parallel direc- 

 tions, but not in contact with each other, giving firmness to the 

 whole of its dorsal portion ; and these are sometimes covered with 

 small tubercles, like the Spicules of Gorgonia (Fig. 233). They 

 may be separated from the soft tissue in which they are imbedded, 

 by means of caustic Potash ; and when treated with dilute acid, 

 whereby the Calcareous matter is dissolved-away, an Organic basis 

 is left, retaining in some degree the form of the original spicule. 

 This basis cannot be said to be a true Cell ; but it seems to be 

 rather a cell in the earliest stage of its formation, being an isolated 

 particle of Sarcode without wall or cavity ; and the close corre- 

 spondence between the appearance presented by thin sections of 

 various Univalve Shells, and the forms of the Spicules of Doris, 

 seems to justify the conclusion that even the most compact Shells 

 of this group are constructed out of the like elements, in a state 

 of closer aggregation and more definite arrangement, with the 

 occasional occurrence of a layer of more Spheroidal bodies of 



