Cephalopoda. MOLLUSCA. -Octopoda. 311 
enveloped in the mantle, which forms a muscular sac, 
raised so as to fill the cavity, is retracted, and a vacuum 
open only in front. The head protrudes from this 
produced ; but when the animal is disposed to let go 
openinf?, is distinct from the body, and has placed 
its hold, the piston is at once pushed forward, and 
round the mouth a number of fle.\ible, fleshy append- 
the vacuum produced by its retraction is in an. instant 
ages, called arms or feet, which serve both as organs of 
destroyed. 
prehension and locomotion. These arms or feet are 
The class Cephalopoda is divided into two orders — 
usually provided with cups or suckers, called acetabula, 
those which have the body naked, and have no shell. 
that act like cupping glasses, and are for the most part 
or onlj^ an internal one ; and those in which the body 
unarmed, though in some instances they are furnished 
is inclosed within an external chambered shell. The 
with a long sharp claw. They form a single or double 
first group, the Naked Cephalopods, have all of them 
senes on the inner surface of the arms, and are various 
only two branchiae, or gills ; whilst the second grouj), 
in number. In those which are unarmed, the median- 
the Shell-bearing Cephalopods, possess four. This 
ism for producing adhesion is extremely interesting. 
essential character has been considered as the best for 
From the margin of each cup, a series of muscular 
dividing them, as one of the families of the first group 
fibres converge toward the centre, at a short distance 
{Argonautidce) is now generally allowed to possess a 
from which they leave a circular aperture, which is 
shell, though not a chambered one ; and another 
occupied by a cone of soft substance rising from the 
{Spirulidce) has a chambered shell, which though cer- 
bottom like the piston of a syringe, and capable of 
tainlj' an internal one, nevertheless in one of the species 
retraction. So perfect is this mechanism, says Dr. 
appears to contain a portion at least of the animal 
Roget, that while the muscular fibres continue con- 
within its last chamber. Rafinesque, in dividing them 
tracted, it is easier to tear away the substance of the 
into two groups or orders, proposed the names of 
limb than to release it from its attachment — it being 
Antepedia and Polarnaxia for them ; but we here adopt 
apparently under the complete control of the animal. 
the nomenclature first established by Professor Owen, 
When the sucker is applied to a surface for the pur- 
who from the number of their gills calls them Dibran- 
pose of adhesion, this piston, having previously been 
chiata and Tetrahranchiata. 
Order I. — DIBRANCHIATA — Two-gilled Cephalopods. 
Animal swimming, having the body naked, the shell 
round, or elongated, and provided with fins ; and 
being either altogether wanting or contained within the 
when the shell is present (except in Argonauta), it is 
body (except in Argonautidce). The head is distinct, 
either horny or shelly, with or without air chambers. 
separate from the rest of the body, and surrounded by 
“ The mantle is usually connected with the back of 
1 
eight or ten fleshy arms, provided with sessile cups or 
the head by a broad muscular band ; but its margin 
suckers. The eyes are sessile, prominent ; the mandi- 
is sometimes free all round, and is supported only 
bles horny, like the beak of a parrot ; and the walls of 
by cartilaginous ridges, fitting into corresponding 
the siphuncle are entire. Gills two in number. Ink 
grooves, and allowing considerable freedom of motion.” 
gland always present. The body of the animal is 
— ( Woodward.) 
Sub-order I.— OCTOPODA (Octopods). 
This sub-order contains three families. The body is 
rounded, and seldom possesses fins ; it is united to the 
head by a broad cervical band, and in the Argonauts 
Family. I. — OCTOPODIDAH — {The Sea Spiders.) 
is covered with a thin, single chambered shell. The 
In this family the arms are all similar, subulate in 
head is large, and the eyes are fixed and incapable of 
form, or conical and tapering, elongated, and united 
rotation. The fleshy arms are only eight in number. 
at the base by a web. The cups are short and sessile. 
and have no swimming membranes. The cups or 
The shell is altogether wanting, or is represented by 
suckers are sessile, flat, and without any horny ring. 
two short styles encysted in the substance of the mouth. 
The branchial chamber is divided longitudinally by a 
— {Owen.) 
muscular partition. “ They are the most eccentric or 
Genus Octopus. — The genus Octopus may be 
‘aberrant’ molluscs, superior in organization to all the 
taken as the type of the family. It has two rows of 
rest, but manifesting some remarkable and unexpected 
cups on the arms, and the body of the animal is round, 
analogies with the lowest classes of animals.” — {Wood- 
and destitute of lateral fins. It is warty, or covered 
icard.) 
with cirrhi, which disappear when in repose, or when 
We have merely space to mention a few of the 
preserved in spirits. The eyes are small, lateral. 
families and genera into which this order is sub- 
covered by the continuation of the surrounding skin, 
divided : — 
and often by one or two transparent eyelids. The 
