Geneeal Chaeacteks. MOLLUSCA. General Characters. celsiii 
tongue covered witli silicious teeth. Jn the Snail 
(Lininea) there are two accessory jaws on the sides. 
Bivalves have a mouth with a pair of soft mem- 
braneous palpi. Many of the higher Mollusks have 
a tongue, or lingual ribbon, which is studded with 
small teeth that are sufficiently hard to break the 
food, 'i’he bivalves have no tongue. A peculiar 
styliform cartilaginous body is found in the stomach 
of some species, which is thought to be the repre- 
sentative of a gizzard ; this has been called the 
gastric dart, and was formerly mistaken for a tongue. 
'I'he liver of Mollusks is always large. 'I'lie renal 
organ is well demonstrated in nearly all species. 
Like the vertebrate intestine, those of the Mollusks 
are longer and convolute in the herbivorous species, 
and short in the carnivorous. In bivalves the in- 
testine is very much convoluted, passing through the 
ventricle of the heart, and terminating near the res- 
piratory aperture, whence the excrements are washed 
away by the water from the gills. 'I'he heart in- 
cludes an auricle, occasionally double, which receives 
the blood from the gills and a ventricle for its pro- 
pulsion into the arteries. 'I'he blood is colorless, or 
pale gray. Respiration is performed under water by 
the Oephalopods, the Acephala and the Brachio- 
pods ; they breathe as the Fishes do, respiring water 
from which the oxygen is absorbed by the gills. 
'I'he Gasteropods are partly air-breathing and partly 
water-breathing; the Land Snails are a familiar ex- 
ample of the former. 'I'he nervous system is repre- 
sented by a ring of ganglia surrounding the throat in 
the Oephalopods and Gasteropods, and in the Ace- 
phala it is situated on the posterior adduct or muscle. 
'I'ouch is the most important sense to these creatures, 
and this resides in every external portion. Special- 
ized organs exist — the tenticles, for example, which 
are situated on the top of the head — and many species 
have numerous tentacular appendages that are sen- 
sitive to touch; these exist in all classes. The en- 
cephalous species seem to be endowed with a sense 
of taste, as their food is selected usually with some 
care. Smell is supposed to be a sense of the uni- 
valves, but not of the bivalves. Sight is an attri- 
bute of the higher forms. Many of the Oephalopods 
have well-developed and very beautiful eyes, some 
being sessile, much resembling those of the Verte- 
brates, and others are on foot-stalks. Some of the 
bivalves have numerous eyes, situated among the ten- 
tacular filaments of the mantle. Hearing is accorded 
to the higher Oephalopods, some having external ears 
and auditory capsules, connected with auditory nerves; 
these capsules contain one or more otolites, or ear- 
bones. Voice is absent, with a very few exceptions. 
The Oephalopods squeak, and emit a sort of groan, 
when removed from the water; and some JSfudibran- 
chiates are credited with the same function. Repro- 
duction is effected in the higher forms by the agency 
of two individuals, 'i'he Oephalopods and nearly all 
of the Marine Gasteropods are of this group. Others 
are moncBcious, the sexes being united in one indi- 
vidual. 'I'he eggs of Marine Mollusks are deposited 
in clusters or in masses upon the rocky bottoms, or 
often upon floating d6bris. Many of them are curi- 
ously formed in a series of chain-like links. A species 
of South American Snail lays eggs of the size of 
those of a Pigeon, which are esteemed as food by 
the natives. 'The economical value of Shell-fish is of 
no little importance. 'I'hough small and seemingly 
insignificant as many are, the enormous numbers o( 
individuals have rendered them an item, collectively, 
that geology shows to be of great significance. 
Recent statements place the number of families of 
Mollmca at three hundred, including between twenty- 
five and thirty thousand living species. About an 
equal number of fossil forms are also known. In the 
language of an author, “ 'I’hey have become the testi- 
mony of the rocks,” and “ 'I'he medals of creation.” 
Wliole mountain ranges are formed of their disinte- 
grated remains. Immense marine deposits are wholly 
the debris of calcareous and silicious shells, the cast- 
off skeletons of as many living animals. As food 
these creatures are equally important, both to man 
and the inferior beasts. From the times of the pre- 
historic “ cave-men ” to the present, the delicious 
bivalve, in variety, has held high rank as an edible. 
'I'he consumption of Oysters alone is simply enormous. 
Among the various manufactures of shell, buttons 
form a large proportion. Cameo-cutting calls for 
great numbers of the marine Couchs. Pearls, both 
precious and the “Mother-of-Pearl,” afford employ- 
ment for great nutnbers in all parts of the world. 
'I'he byssus, or silky floss, seen attached to Mussels, 
and by which the shell fastens itself to other objects, 
is utilized by the Italians, who spin it into delicate 
hosiery, which resembles the finest silk. From the 
remotest time Whelks have been notable as beai'ers 
of the Tyrian Dye. Several well-known species of 
shells upon our own coast emit a similar brilliant 
purple fluid. 'I'he growth of shells is often very 
rapid. 'I'he Land Snails are mostly biennial ; they 
are hatched in the summer and autumn, and by win- 
ter time they are half grown, and in a year’s time 
they have attained full size. Garden Snails have 
lived in confinement eight years. Many of the bi- 
valves, as the Mussel, according to AMoodward, 
attain their full growth in one year. 'I’he Oyster 
continues four or five years and then ceases to grow 
outwards; after this the shell continues to thicken. 
Some Mollusks are notable for tenacity of life. Mr. 
Gray, of the British Museum, received a living Pond 
Mussel from Australia, which had remained out of 
water more than a year. Pond Snails are often 
found in mahogany which is brought from Honduras. 
One naturalist in Calcutta found shells living in his 
cabinet that had been there five years. 
The permanency o^Mollusca is well provided for by 
their fecundity. The spawn of a single doris is esti- 
mated by Darwin to contain six hundred thousand 
eggs; a River Mussel produces three hundred thou- 
sand young in a season. 'I’he aquatic species have 
many enemies, or they have a numerous following 
that look to them for subsistence. 'I'he terrestrial 
Shell-fish are less exposed and wisely planned. 'I'hey 
lay fewer eggs; considerable instinctive care is evi- 
dently taken of them. 'I'he great bird-like eggs of 
the tropical Bulimi are placed carefully within leaves 
