CUTTLE-BONE. 509 



the soft parts of the Calamary, to which, in fact, they are so little ad- 

 herent that they fall out as soon as the sac wherein they are secreted 

 is laid open. 



(1527.) In the Cuttle-fish (Sepia officinalis) the dorsal plate (os 

 Sepice) is found in the same situation as the gladius of the Calamary, 

 from which, however, it differs remarkably both in texture and com- 

 position. The cuttle-bone, with the appearance of which every one is 

 familiar, is principally composed of calcareous substance, and, were we 

 to judge of its weight from its bulk, would seem calculated materially 

 to interfere with the movements of an aquatic animal destined to swim 

 about, and consequently needing whatever assistance might be derived 

 from lightness and buoyancy. Did a creature so apparently destitute 

 of natatory organs possess a swimming-bladder like that of a fish, to 

 assist in supporting it in the water, we should conceive such an ap- 

 paratus to be far more adapted to its predatory habits than a shell so 

 bulky as that which it is destined to carry. 



(1528.) We have, however, already seen, in the case of the Nautilus, 

 that it would be by no means impracticable to convert a shell into a 

 float nearly equalling a swimming-bladder in efficiency ; and on more 

 accurate examination it becomes obvious that even in the bone of the 

 Cuttle we have a provision of a similar nature, though the end arrived 

 at is obtained in a very different manner. On making a section of a 

 cuttle-bone, it will be found to be composed of numerous stages of very 

 thin calcareous plates placed at some distance above each other, and 

 kept apart by the interposition of vertical laminae of the same substance, 

 having, from the tortuosity of their meanderings, the appearance of 

 millions of microscopic pillars. Thus organized, the shell in question 

 becomes sufficiently light to float in water, and consequently, from 

 its buoyancy, no doubt assists, instead of impeding, the movements of 

 the mollusk. This admirable float, like the horny gladius of Loligo, is 

 lodged in a membranous capsule and enclosed in the back of the Sqoia, 

 having no connexion whatever with the sides of the cavity wherein it 

 is placed, being so loosely adherent that it readily falls out on opening 

 the sac. 



(1529.) The cuttle-bone is formed in the same manner as other shells, 

 by the continued addition of calcareous laminae secreted by that side of 

 the containing capsule which is interposed between the shell and the 

 abdominal viscera ; and these layers, being successively added to the 

 ventral surface of the shell, thus gradually increase its bulk as the 

 Cuttle-fish advances to maturity. Neither in the mode of its growth 

 nor in its texture, therefore, does the os Sepice resemble bone, properly 

 so called ; it receives neither vessels nor nerves, but is in all respects 

 a dermal secretion, imbedded in the mantle, and formed in the same 

 manner as the dorsal plate of the Slug. 



(1530.) We now come to consider the long-disputed question relative 



