BOOK VI. SHELL-FISH, LAMP-SHELLS, SEA. 
URCHINS, STAR-FISHES, MOSS-ANIMALS. 
WORMS, CORALS, ¥ELLY-FISHES, 
wND SPONGES 
BY W. SAVILLE-KENT, F.L.S., F.Z.S. 
CE ACP) Boe, ok 
SHELL-FISH, OR MOLLUSCS 
HE Molluscan Group or Sub-kingdom represents 
one, if not the most important, of the invertebrate 
sections of living animals with relation both to its 
numbers and variety and in its commercial and economic 
utility to mankind. In its ranks are included all those 
animals generally known as Shell-fish, and familiar to the 
non-scientific in the shape of Oysters, Mussels, Whelks, 
Periwinkles, and the innumerable varieties of gorgeous or 
delicately tinted shells of tropical seas. 
Collectively, Molluscs differ from all such invertebrate 
groups as Insects, Crustaceans, and Worms in that they 
possess neither jointed limbs nor jointed bodies, their 
body-substance being enclosed by a more or less distinct 
muscular sac, or integument, technically known as the 
“mantle.” Molluscs possess no internal skeleton; but for 
the protection of their soft and otherwise defenceless 
bodies the mantle is among the great majority of species 
endowed with the property of secreting a more or less 
indurated calcareous shell, within which, when danger 
threatens, the creature can entirely withdraw. In some 
species the shell secreted is relatively small, and serves 
only as a protective shield to especially vital areas; while 
in a third very considerable assemblage a shell is altogether 
absent. The minute yet technically recognisable structural 
differences between the shells of even the most closely 
allied specific forms, and the wider and distinctly evident 
divergences that separate the more remotely connected 
varieties, furnish the basis for their classification and 
nomenclature by the systematic conchologist. Molluscan 
shells, being so extensively preserved in the fossil state, 
furnish the geologist with invaluable data for his deter- 
mination of the age and respective relationship of the 
fossil-bearing strata of the earth’s crust. 
Having no jointed limbs, molluscs are dependent upon 
some other mechanical adaptation for their powers of loco- 
motion. This, in the majority of species, is represented by 
a modification of the lower surface of the animal’s body, 
339 
Photo by W7, Saville-Kent, Fd. 5. 
AN OCTOPUS CROUCHING 
IN A ROCK-POOL 
Green shore-crabs constitute the chief food of 
the octopus 
My A Pen { 
Photo by WH’. Sa -Kent, 
AN OCTOPUS ON ITS BACK AT 
BAY, LEFT HIGH AND DRY BY 
THE RETREATING TIDE 
In this attitude the octopus can use its many-suckered 
tentacles and its formidable parrot- 
like beak as defensive weapons 
