494 
MOLLTJSCA. 
This term is derived from two Greek words, Jcephale^ a head, and pous, a foot, and refers to the 
fact that the animals of this class have the legs arranged around the head, and may therefore he 
called head-footed. These legs, it is true, are also arms, and being furnished with rows of suck- 
ing-cups, enable these animals to maintain a firm grasp of any object on which they lay hold. 
They breathe by means of branchite connected with a funnel or siphon, through which the water 
passes off. This arrangement answers a double purpose, for by expelling the water vigorously 
through this siphon, they propel themselves rapidly along, this being their common mode of 
swimming. They thus move backward, their eyes, however, being so situated as to command a 
view of the objects in their course. Some species possess fins, and some use their arms to aid 
them in swimming. In all the body is inclosed in a sac-like mantle, generally of thick, leathery 
skin, covered, however, Avith a delicate cuticle, which in some species displays the most brilliant 
colors. They are all oviparous. This class is divided into two orders, the Dibranchiata and 
Tetrahranchiata. 
ORDER 1. DIBEANCHIATA. 
This term, derived from the Greek dis, two, and the Latin hrancMce, gills, signifies animals 
having two gills. The order includes a great number of fossil as well as living species, dis- 
tinguished for possessing two branchiae, and a structure of body which enables them not only to 
swim rapidly through the water, but to creep with the head downward, upon the bottom of the 
sea. The shell is generally 
internal, and often merely 
rudimentary ; if external 
it is not chambered. The 
arms are eight or ten in 
number, and furnished 
with sucking-discs, and 
the body is often sup- 
plied with a pair of fins. 
They are divided into two 
sections, the Octopoda and 
Deccqyoda. 
THE OCTOPODA. 
These are furnished with 
eight arms or legs ; two 
of which, in one genus, 
enable the animals to form 
shells, to which, however, 
the body is not attach- 
ed; others are naked, 
having a slight trace of an 
internal shell. They are 
for the most part exceed- 
ingly active and voraci- 
ous, and prey with avidity 
upon Crustacea and fishes. 
They are provided with 
a sac containino- a black 
and acrid fluid, which they eject into the water so as to obscure it and render it offensive, by 
means of which they escape from their enemies. 
ARGONAUT "WITH THE SHELL. 
