CEPHALOPODA. 15 
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 
Loligide, the Loligopside, and the Teuthide, they assume the form of a horny plate, 
termed the g/adius, which in some genera is thin and feather-shaped, or more or less 
spatulate, lanceolate, or ensiform; and in others, they are elongated, narrow, and termi- 
nated posteriorily by a simple cup-shaped appendage. In the Sepzde the shell presents 
aseries of thin calcareous plates, not siphoniferous, but separated by numerous exceed- 
ingly minute pillars, and forming a convex mass terminated by a mucro or spine; in 
the Belemnitide 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 Spirulide, 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 Paleeontologist 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 Loligide, Loligopside, and Teuthide, 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 
Sepide and Belemnitide, 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 Sepidz, the most littoral of 
all the Cephalopods: to the deep-sea swimmers it is denied; it would in fact be 
