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Classification. MOLLUSCA. General Characters. 
Sub-kingdom — MOLLUSCA. 
SUB-BRANCH.— MOLLUSCOIDEA. 
'J'he term Mollusca is used to designate the second 
of the five grand divisions or structural types of 
tlie Animal Kingdom. 'I’he science which treats 
of Shell-fish — shells and their living inhabitants — is 
called Oonchology. 'I'he soft parts were almost un- 
known to the ancients ; consequently the shells, or 
exterior coverings, were studied and classified inde- 
pendently of the living animal. Mollusks, or Shell fish, 
as they are collectively called, are divided into sev- 
eral great branches. The best approved arrangement 
is that adopted by Dr. 'I'heodore Gill, in his Families 
of Mollusks, which accords with the system and col- 
lections of the Smithsonian Institution. It is as 
follows : 
Class A.— CEPHALOPODA. 
Order I. — Dibkanchiata. 
Hub-order -OcTOTOjyx. 
Fmt,r Families. 
Hub-order — Sepiophora 
Order II. — Tetkabranchiata. 
Class B. -GASTEROPODA. 
Subclass— DIGECA. 
Order III.— Pectinibranchiata. 
Hub-order — Toxoglossa. 
Hub-order — Rachiglossa. 
Hub-order — Tcenioglossa. 
Hub-order — Ptenoglossa. 
Order IV. — Heteropoba. 
Order V.— Ruipidoglossa. 
Podopthalma. 
Hub-order — Dicranobranchia. 
Order VI. — Docoglossa. 
Hub-order — Proteobrancuia. 
Hub-order — Abranciiia. 
Order VII. — Polyplacophora. 
Sub-class— PULMONIFERA. 
Order VIII. — Pulmonata. 
Hub-order — Geophila. 
Hub-order — Bassomatophora. 
Subclass— OPISTHOBRANCHIATA. 
Order IX. — Tectibranchiata. 
Order X. — Nudibranciiiata. 
Hub-order — Pygobranchia. 
Hub-order — Polybranciiia. 
Hub-order — Pelubrancuiata. 
Subclass— PTEROPODA. 
Order XI. — Thecosomata. 
Order XII.— Gymnosom ata. 
Sub-class— PROSOPOCEPHALA 
Order XIII.-Solenoconcuhs. 
Class C.— CONCHIFERA. 
Order XIV. — Dimyaria. 
Order XV.— Metarrhipt.e. 
Order XVI. — Heteromyaria. 
Order XVII. — Monomyaria. 
Order XVIII. — Rudista. 
Class D.— TUNICATA. 
Order XIX.— Laccobranchia. 
Order XX.— Dactylobranchia. 
Order XXI.— Tjeniobranchia. 
Order XXII.— Larv alia. 
Class E.— BRACHIOPODA. 
Order XXIII. — Arthropomata. 
Ordei- XXIV. — Lyopomata. 
Class F.— POLYZOA. 
Order XXV.— Phylactol^mata. 
Order XXVI.— Gymnol^mata. 
Order XXVII (?).— Rhabbopleur.®. 
Most Mollusks are enclosed in an external shell ; 
the acephalous forms are always thus provided, the 
shell being in two pieces; hence the term. Bivalve. 
The Gasteropods have one covering of shell, and are 
designated Univalves. A few of the latter have no 
shells whatever. Others have a more or less perfect 
internal shell, secreted beneath the mantle, as in the 
naked snails and slugs. In the egg of a Mollusk the 
nucleus of the shell is seen to cover the young animal 
even before the internal organs begin to assume de- 
finite form, and is noticeable from its transparent horn 
color. After the birth of the Mollusk, the growth 
of the shell is promoted by additions of carbonate 
of lime and animal matter to the lip or circumfer- 
ence of the aperture. This is effected by secretion 
from the edges of the mantle which lie in con- 
tact, the soft deposit becoming calcified on exposure 
to the air. 'I’he external skin or epidermis which 
covers the shell, as well as its cellular substance, is 
exuded from the collar or neck of the animal, the 
more delicate porcelain-like interior and “ Mother of 
Pearl ” being secreted from the interior or visceral 
membranes. Analysis has revealed the presence of 
sulphuric and muriatic acids in the saliva of some Mol- 
lusks. This fact sheds a degree of light upon the 
hitherto vexed questions regarding the boring habits 
and destructive powers of these animals. The oper- 
culum, seen in univalves, is a horny plate, sometimes 
quite thickened by a deposit of lime or shell, and 
adapted to cover the opening when the animal is 
entirely within. It is a means of defense, and in 
some of the large Conchs it is a falchion-like weapon, 
capable of considerable mischief. 'I’he shell of Mol- 
lusks may be considered their external skeleton. 
We find certain muscles attached, mainly those to 
serve the purpose of closing the valves ; yet others, 
in the bivalves, are seen to control the movement of 
the mantle. In the higher Cephalopods the muscles 
quite resemble those of the Vertebrates, being at- 
tached to cartilages, representing the attachment to 
bones. Cephalopods are furnished with horny jaws, 
with which they secure their food. 'I’he Helices 
have an upper jaw only, which is opposed to a 
