i^lO Intkoductiok. MOLLUSCA. Arrangement. 
other centres, connected with the lower side of the 
esophageal ring, send nerves to the foot, viscera, and 
respiratory organ.” — ( Woodward^ 
AERANGEMENT OF MOLLUSCA. 
Previous to the time of Cuvier, the animals which 
we now know under the name of Mollusca, were con- 
fusedly arranged, and very imperfectly known. They 
“ were intermingled with worms and with zoophytes, 
while a great number of them stood detached from 
their allies under the ordinal designation Testacea, 
merely because they were inclosed in hard calcareous 
shells ; the knowledge of the inferior tribes being then 
too little advanced to admit of the application of any 
characters but those that were derived from exterior 
form and consistence. By his numerous careful dissec- 
tions, Cuvier was early enabled to detect and appre- 
ciate the unnaturalness of the prevalent systems; and 
when his labours had convinced him that their over- 
throw was necessary to the progress of science, they 
had at the same time furnished him with the materials 
out of which he sought to erect a new system, which 
has been of incalculable advantage to scientific concho- 
logy, and which remains untouched in all its grand 
lineaments, though his successors have certainly im- 
proved and worked out many of the minor details.” — 
(Johnston.) It is to Cuviei', then, that we owe the 
grouping together the animals to which the name of 
Mollusca is now properly restricted. 
This great naturalist took as the first division the 
possession or the want of a head, and arranged the 
various classes into two principal groups : — 1st, Those 
which had a head more or less distinctly formed, the 
Encephala; and 2nd, Those which were destitute of 
that important appendage, the Accphala. In the first 
group he placed the Cep/halopoda, or Cuttle-fishes ; the 
Fteropoda, Winged or Floating mollusca ; and the 
Gasteropoda., or Snails. In the second group he placed 
the Conchifera, or Bivalve shells; the Brachiopoda, 
Brachiopods, or Lamp shells ; and the Cirrhopoda, or 
Barnacle shells. This last class is now ascertained to 
belong to the Crustacea, but may be replaced by the 
Tunicata, or Tunicaries — which he made merely an 
order of the AcephalaT Various arrangements have 
been suggested by naturalists since Cuvier put forth 
his system, but we will only mention one, that of Dr. 
Gray ; merely premising that however varied the 
methods propounded have been, they all hinge in 
reality upon that of the celebrated author of the “Eegne 
Animal.” 
Cuvier divided the Mollusca into two great groups, 
according as they possessed or as they wanted a head. 
Dr. Gray, in adopting the classes of Cuvier, divides 
them into two great groups also, but takes as the basis 
of his arrangement the possession or want of a foot. As 
this arrangement is the one adopted in our great national 
collection in the British Museum, and has the merit ot 
being patent to all who wish to study Conchology ; and 
as we shall have frequent occasion to refer to it in the 
following pages, we subjoin it as follows : — 
^ SUB-KINGD(W MOLLUSCA. 
Group I. — Pedipera, crawling on a toot placed under the 
body. 
Classl. — Gasteropoda— Gasteropoda, Univalves,o\-Snails. 
Head distinct, with eyes and tentacles ; body usually 
protected by a conical, more or less spiral shell, often 
furnished with an operculum. 
Class 2. — CoNCHiFERA— (7o?ic7t?yers or Bivalves. 
Head indistinct ; mouth placed between the gills ; they 
and the body inclosed between the two leaves of the 
mantle, which are covered by two shelly valves united 
above by a ligament. 
Group II. — Apoda. Foot none, or very rudimentary. 
Class 3. — Brachiopoda — Brachiopods or Lamp Shells. 
Mouth placed at the base of two spirally-twisted 
ciliated arms between the two leaves of the mantle, 
which are covered with two separate shelly valves ; they 
live attached to other marine bodies. 
Class 4. — Pteropoda — Pteropods or Floating Mollusca. 
Head prominent, with one or rarely two pairs of fins 
on the side of the neck, by means of which they swim 
on the ocean; body often covered with a thin conoidal 
shell. 
Class 5.— Cephalopoda — Cephalopods or Cuttle-fishes. 
Head distinct, large, with eight or ten or more arms 
by which they walk head downwards. 
To these we must add — 
Class 6. — Tunicata — Tunicated Mollusca or Tunicaries. 
No distinct head ; mouth at the bottom of the bran- 
chial sac ; body protected bj' an elastic gelatinous tunic, 
with two orifices, one for the admission of water and 
food into the interior, and the other for the e.\pulsion 
of excrementitious matters ; no shell. 
As in the preceding parts of this work, the plan 
followed has been, in general, to commence with the 
most highly-organized forms, descending to those lower 
in the scale, we will deviate here from the sketch just 
given of the arrangement in the British Museum, and 
begin with the Cephalopods. “ The type of structure 
which characterizes the Dibranchiate order of Cepha- 
lopods,” says Professor Owen, “ places these mollusca 
not only at the head of that division of the animal 
kingdom, but in respect to its closer proximity to the 
vertebrate type, unquestionably at the head of the 
whole invertebrate series ” Premising this, we will 
therefore commence w'ith — 
Class I.— CEPHALOPODA — Head-walking Molluscs. 
The Cephalopods are the most highly organized of all 
the molluscs. They are symmetrical animals, having 
their right and left sides equally developed. They 
have a large distinct head, a mouth armed with power- 
ful horny jaws, acting vertically like the mandibles of 
birds ; a large and fleshy tongue, and large well-devel- 
oped eyes, approaching in structure those of verte- 
brated animals. The body, which is soft and fleshy, is 
