516 MOLLUSCOUS ANIMALS GENERALLY. 



teeth of Rodentia. 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 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 cell-like bodies, generally somewhat hexa- 

 gonal in form, and consolidated by a deposit of calcareous 

 matter, which is sometimes so arranged as to be quite transpa- 

 rent, whilst in other instances it presents an appearance closely 

 resembling that delineated in Fig. 255. 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 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 ( 309). They 

 may be separated from the soft tissue in which they are imbed- 

 ded, 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 correspondence between appearances 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 the same kind, like those forming the rudi- 

 mentary shell of Limax. 



343. The animals composing the class of Cephalopoda (Cuttle- 

 fish and Nautilus tribe), are for the most part unpossessed of 

 shells ; and the structure of the few that we meet with in the 

 genera Nautilus, Argonauta (Paper-nautilus) and Spirula, does 

 not present any peculiarities that need here detain us. The 

 rudimentary shell or sepiostaire of the common Cuttle-fish, how- 

 ever, which is frequently spoken of as the "cuttle-fish bone," 

 exhibits a very beautiful and remarkable structure, such as 

 causes sections of it to be* very interesting microscopic objects. 

 The outer shelly portion of this body consists of horny layers, 

 alternating with calcified layers, in which last may be seen an 

 hexagonal arrangement somewhat corresponding with that in 

 Fig. 255. The soft friable substance that occupies the hollow 

 of this boat-shaped shell, is formed of a number of delicate 

 plates, running across it from one side to the other in parallel 

 directions, but separated by intervals several times wider than 



