CHAPTER I.
SHELL-FISH, OR MOLLUSCS.
The 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.
Photo by W. Saville-Kent, F.Z.S., Milford-on-Sea.
AN OCTOPUS CROUCHING IN A ROCK-POOL.
Green shore-crabs constitute the chief food of the octopus.
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 determination of the age and respective relationship of the fossil-bearing strata of the earth's crust.
Photo by W. Saville-Kent, F.Z.S., Milford-on-Sea.