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MOLLUSCA. 
THE ELEPHANT S TOOTH. 
THE DENTALIID^. 
These, wliich. have been called Tooth-Shells and Shell- Worms, have a tube-shaped shell, taper- 
ing from one end to the other, and 
slightly curved throughout, so as to 
appear like a minute elephant's 
tusk; hence the common species, 
Dentalium elephantinum^ is popu- 
larly called Elephants Tooth. It 
has an aperture at each end ; the animal inhabiting it is of a cylindrical form, and inclosed in a 
sac-like mantle. Thirty living and seventy fossil species are known. They are of various sizes, 
and are found on nearly all shores. 
THE OPISTHOBRAITCHIATA. 
This term is from the Greek opisthos, behind, and hranchice, gills. In the animals which com- 
pose the group, the branchiae are not generally inclosed within a cavity of the mantle, but are 
more or less exposed on the back or sides of the animal, generally toward the posterior portion ; 
and the auricle of the heart, which receives the blood from the gills, is placed behind the ven- 
tricle. All the species are hermaphrodites ; few of them are inclosed in a shell ; some have 
an internal shell, but the majority are naked. The three following families are called JVudi- 
branchiata, the others Tectihranchiata. 
THE DORIDyE. 
In these animals, called Sea- Lemons., the body is shelless and oblong ; the gills plume-like, and 
placed in a circle on the middle of the back ; the tentacles two ; the eye-specks immersed behind 
the tentacles, and not always visible in the adult. They are delicate little creatures, adorned 
with pleasing colors, and are generally found along the sea-coasts, crawling upon sea-weed. 
They vary in size from one quarter of an inch to three inches, are carnivorous, and feed on zoo- 
phytes and sponges ; found on rocky coasts in nearly all parts of the world. 
THE TRITONIID^. 
The Tritonias are naked, like the preceding, with plumose or 
papillose gills ; the body is elongated, sometimes six inches long ; 
some are found under stones at low water, and some floating on 
sea-weed. The T. Homhergii attains a length of six inches. 
There are several analogous genera. 
THE ^OLIDIDJE. 
In these animals, which generally appear like small slugs, the branchiae are arranged along on 
each side of the back; the tentacles 
are not retractile; some species 
have a very singular appearance, 
the body being slender, and the 
gill-tufts supported at the extremi- 
ties of lateral footstalks. 
THE PLEUROBRANCHID^. 
These animals, called Umbrella- Shells., are furnished 
with a shell, which, however, is often concealed by 
the mantle ; the foot is usually large, and the feather- 
like gill hidden between a fold of the mantle and the 
foot. The shell is sometimes limpet-shaped. The 
Umbrella Mediterranea and the U. Indica are the 
only known species. They are found on rocky coasts, 
TEITONIA HOMBERGII. 
THE EOLIS PAPILLOSA. 
THE MEDITEEKANEAN UMBRELLA-SHELL, 
at various depths from the sm-face to thirty fathoms. 
