A CUTTLE-BONE. 45 



neighbours in the animal scale, and shows that they 

 are true shell-fish by right of birth and by all the 

 lawful titles which a distinct zoological position can 

 confer. Bones, it is true, they have none, for these 

 structures are the exclusive possession of the back- 

 boned group ; hence, if the limy plate found in the 

 canary's cage proved (as prove it did) to be a cuttle- 

 fish belonging, it is clear that to call it a " bone " 

 was a contradiction in terms. 



The limy plate, in truth, is not a bone, but a "shell." 

 Unlike a shell in every respect, it nevertheless duly 

 represents, in the sepia-cuttlefish, the familiar structure 

 we see in the snail or the oyster. We know this, 

 first of all, because it is made by that part of the 

 animal which in other molluscs, and also in other 

 cuttlefishes, manufactures the shell. This is the outer 

 layer, or integument of the body, which, in natural 

 history language, we call the " mantle." 



Whatever structures this layer forms and secretes 

 are " shells," in the true sense of the term. The 

 objects it manufactures, like the shell of the sepia- 

 cuttlefish itself, may be utterly unlike shells. As 

 such, they may not be recognised at all ; yet in their 

 nature they are shells nevertheless. If we turn for a 

 moment to two other cuttlefishes, we may be able to 

 prove this assertion very easily. 



There is a cuttlefish, extremely rare indeed as a 

 living animal, but whose shells, as I have remarked, 

 are common enough in drawing-rooms. This is the 

 pearly nautilus (fig. 14). It is like the " last of the 

 Mohicans," in that it is the sole survivor of a once 

 large group of shell-possessing four-gilled forms. Its 

 shell is of very perfect character. It is divided into 

 compartments, each of which the animal successively 



