98 
ROOM XIII. lusca, which are characterized by having a series of co- 
Nat. Hist, nical arms radiating round the mouth, which serve as 
organs of motion and prehension: some that have 
eight arms have no shell, as Octopus, Eledona, and 
Ocythoe: others have ten arms, two of which are longer 
than the rest, and the remaining eight placed on the 
sides of the body. These have either an internal bone, 
as the cuttle fish, (sepia,) or a horny plate called the 
Sea Pen, as the Loligo , Sepiola, and Cranchia. Lastly, 
some have many short tubular retractile arms, and these 
inhabit the last division of an external many chambered 
shell, as the Nautilus; to these are allied the genera 
Orthocerites , Ammonites , Scaphites , Turrilites, and 
also probably the Belemnites , all of which are fossil. 
In Cases 3 and 4 are placed a series of models on 
an enlarged scale, and some specimens of minute 
bodies, which have been regarded as analogous to 
the Nautili , but the nature of the animals is not 
known; they probably belong to several different Or¬ 
ders. Some have supposed them to be internal shells, 
but this cannot be the case with all, as many are at¬ 
tached by their outer surface to sea-weed and shells. 
They are formed of cells, furnished with one or more 
small mouths, placed one on another in different direc¬ 
tions, some forming straight lines, as Nodosaria, and 
others, spiral ones, as Rotalia . In others the cells are 
half the length of a whorl, so that each new cell changes 
the situation of the mouth from one to the other end of 
the shell, as in the Miliolce: and in others the cells are 
divided into numerous longitudinal tubes, as in Alveo- 
lina and Fabularia. 
In Cases 5 to 60 are arranged the shells of the Gas- 
teropodous Mollusca—which walk on a broad, flat, 
ventral disk. All these animals have a single spiral 
shell, except those belonging to the genus Chiton, 
which have a series of valves down the back. The 
Gasteropoda are divided into orders according to the 
form of their respiratory organs. 
The greater number of those furnished with shells 
have 
