BOOK VI. SHELL-FISH, LAMP-SHELLS, SEA- 



URCHINS, STAR-FISHES, MOSS-ANIMALS, 



WORMS, CORALS, JELLT-FISHES, 



AND SPONGES 



BY W. SAVILLE-KENT, F.L.S., F.Z.S. 



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. 



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 



Phtit n If. SavitU-Ktni, F.2.S. 



AN OCTOPUS CROUCHING 

 IN A ROCK-POOL 



Green shore-crabs constitute the chief food of 



Phtl, tj If. Sj:i'.'.--Kint, F.Z.S 



AN OCTOPUS ON ITS BACK AT 



BAY, LEFT HIGH AND DRY BV 



THE RETREATING TIDE 



/ this attitude the octopus can use its many-suckcrcd 

 tentacles and its formidable parrot- 

 like beak as defensive 'weapons 



