MOLLUSCOUS 'ANIMALS GENERALLY. 179 



features indistinct, since the residuum left after the removal of the cal- 

 careous matter is usually so imperfect, as to give no clue whatever to the 

 explanation of the appearances shown by sections. Nevertheless, the 

 structure of these shells is by no means homogeneous, but always exhibits 

 indications, more or less clear, of a definite arrangement. The ' porcel- 

 lanous' shells are composed of three layers, all presenting the 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 assemblange of thin 

 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 laminae 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 inter- 

 sect 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 

 Cyprc&a 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. 



573. 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, Limax rufus, a thin 

 oval plate of calcareous texture is found imbeddedln 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 it presents an appearance 

 closely resembling that delineated in Fig. 390. In the epidermis of 

 the mantle of some species of Doris, on the other hand, we find long 

 calcareous spicules, generally lying in parallel directions, but not in con- 

 tact with each other, giving firmness to the whole of its dorsal portion; 

 and these are sometimes covered with small tubercles, like the spicules 

 of Gorgbnia (Fig. 363). 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 parti- 

 cle of sarcode without wall or cavity; and the close correspondence 

 between the appearance presented by thin sections of various Univalve 

 shells, and the forms of the spicules of Doris, seems to justify the con- 

 clusion 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 the same kind, like those forming the rudimentary shell of 

 Limax. 



574. The structure of Shells generally is best examined by making 

 sections in different planes as nearly parallel as may be possible to the 

 surfaces of the shell, and other sections at right angles to these: the 



