CEPHALOPODA. 1 5 



external shells ; but they are provided with internal horny or calcareous substances, 

 encysted in the back of the mantle, and frequently not in any way attached to the 

 animal, but loose in the cells containing them. In the naked Octopods these internal 

 substances are of the simplest form, and consist of two short, horny, gelatinous styles. 

 Among the Decapods, they become gradually more complicated in structure. In the 

 Loligida, the Loligop&ida, and the Teuthida, they assume the form of a horny plate, 

 termed the gladiiis, which in some genera is thin and feather-shaped, or more or less 

 spatulate, lanceolate, orensiform; and in others, they are elongated, narrow, and termi- 

 nated posteriorily by a simple cup-shaped appendage. In the Sep idee the shell presents 

 a series of thin calcareous plates, not siphoniferous, but separated by numerous exceed- 

 ingly minute pillars, and forming a convex mass terminated by a macro or spine ; in 

 the Belemnitida it consists of a chambered cone perforated by a siphuncle, and lodged 

 in a cavity formed in the upper portion of a calcareous rostrum, more or less pointed 

 or obtuse ; and in the Spinilidce, the sole remaining family, it is a calcareous, 

 horizontally convolute, multilocular, and siphonated shell, with distinct whorls, and 

 imbedded in the animal, but having portions of the last whorl merely covered by the 

 outer layers of the skin. These differences in structure appear to be always accom- 

 panied with distinct zoological forms ; and hence the Palaeontologist is enabled to form 

 a tolerably correct judgment of the analogy between the existing species and those 

 which inhabited the ancient seas, although the testaceous remains are, most frequently, 

 the only means of comparison afforded to him. 



These internal shells are formed by secretions, from the internal surfaces of the 

 cells, of a horny or calcareous substance, which is deposited in successive layers, and 

 by the continual addition of which they increase in size as the growth of the animal 

 proceeds. Their functions are various, and in accordance with their particular 

 structure. When the internal shell is gelatinous or horny, as in the Octopoda, and in 

 the Loligida, Loligopsidce, and Teuthida, the function is chiefly to support and 

 strengthen the body, analogous with that of the bones in the vertebrate animals. It 

 appears that the greater or less length of the shell has always relation to the 

 swimming power of the animal. When the internal shell is horny or calcareous, and 

 contains parts filled with air, as is the case in the several other decapodous families, it 

 acts as a float ; and in this function, like the external shell of the tetrabranchiate 

 Cephalopods, it represents the swimming bladder of fish ; but the volume of air 

 contained within the shell is, apparently, in an inverse ratio with the swimming 

 power of the animal. In addition to these functions, the internal shells, which 

 are provided with a mucro or rostrum at their posterior extremities, as in the 

 Sepida and Belemnitidce, are enabled by its means to break the force of the shocks 

 caused by the body striking against any hard substance in its retrograde motion. In 

 the recent Cephalopods this protection is confined to the Sepida, the most littoral of 

 all the Cephalopods : to the deep-sea swimmers it is denied ; it would in fact be 



