CEPHALOPODA. 441 



of the gladius as the growth of the animal proceeds. Several of 

 these plates may be produced in succession, and in old individuals 

 it is not uncommon to find two or three enclosed in the same 

 cavity, and placed one behind the other ; that nearest the visceral 

 aspect of the chamber being the most recently formed. These 

 rudimentary shells have no connection whatever with the soft parts 

 of the Calamary, to which, in fact, they are so little adherent that 

 they fall out as soon as the sac wherein they are secreted is laid 

 open. 



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

 is found in the same situation as the gladius of the Calamary, from 

 which, however, it differs remarkably both in texture and compo- 

 sition. 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 assist- 

 ance might be derived from lightness and buoyancy. Did a crea- 

 ture so apparently destitute of natatory organs possess a swim- 

 ming-bladder like that of a fish, to assist in supporting it in the 

 water, we should conceive such an apparatus to be far more adapted 

 to its predatory habits than a shell so bulky as that which it is 

 destined to carry. 



(488.) We have, however, already seen in the case of the Nauti- 

 lus 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 mil- 

 lions 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 Sepia, having no connection whatever with the sides 

 of the cavity wherein it is placed, but so loose that it readily falls 

 out on opening the sac. 



(484.) The cuttle-bone is formed in the same manner as other 



