Philodina roseola.—(Highly magnified.)
a. Respiratory tube.
b. Alimentary canal.
c. Cellular mass.
d. Terminal intestinal pouch.
e. Anal orifice.
The rotifera are either naked or covered with a sheath, and many inhabit a tube formed by themselves, attached by its lower end to some water-plant, and open at the summit, from which the animal protrudes when it would exercise its active instincts, and into which it retires for repose from labour or for refuge from alarm. The majority, however, have a furcated foot, which is often capable of contraction by a set of telescopic sheathings or false joints, and by which they are enabled to secure a hold of the minute stems of water-plants. This is their ordinary position when keeping their wheels in action for a supply of food or of water; but they have no difficulty in letting go their hold, and either creeping along by alternate contractions and extensions or swimming away in search of a new attachment. From the neck projects a telescopic spur, supposed to be an organ of respiration, and just below this are seen two minute red specks, supposed to be eyes. The first rotifer was discovered by Leeuwenhoek, in 1702; now more than 180 species are known, and new discoveries are constantly adding to their numbers. They are chiefly found in sweet water, but some are inhabitants of the sea, as, for instance, the Synchæta baltica, remarkable for its luminous powers. It measures about 1/125 of an inch in length, and but 1/350 in width, so that it is invisible to the sharpest unassisted sight: but when viewed through a microscope, it appears as a beautiful and richly organised creature, clear as glass and perfectly colourless, except that its stomach is usually distended with yellow food, and that it carries a large red eye, which glitters like a ruby.
"Its motions too," says Mr. Gosse, "are all vivacious and elegant. It shoots rapidly along or circles about in giddy dance, in company with its fellows, sometimes near the surface, at others just over the bottom of the vase in which it is kept. Occasionally the foot with the tiny toes is drawn up into the body and then suddenly thrown down, and bent up from side to side as a dog wags his tail. Sometimes the rotatory organs are brought forward and then spasmodically spring back to their ordinary position, when the little creature shoots forward with redoubled energy. In all its actions it displays vigour and precision, intelligence and will."
[CHAP. XV.]
MOLLUSCS.
The Molluscs in general.—The Cephalopods.—Dibranchiates and Tetrabranchiates.—Arms and Tentacles.—Suckers.—Hooked Acetabula of the Onychoteuthis.—Mandibles.—Ink Bag.—Numbers of the Cephalopods.—Their Habits.—Their Enemies.—Their Use to Man.—Their Eggs.—Enormous size of several species.—The fabulous Kraken.—The Argonaut.—The Nautili.—The Cephalopods of the Primitive Ocean.—The Gasteropods.—Their Subdivisions.—Gills of the Nudibranchiates.—The Pleurobranchus plumula.—The Sea-Hare.—The Chitons.—The Patellæ.—The Haliotis or Sea-Ear.—The Carinariæ.—The Pectinibranchiates.—Variety and Beauty of their Shells.—Their Mode of Locomotion.—Foot of the Tornatella and Cyclostoma.—The Ianthinæ.—Sedentary Gasteropods.—The Magilus.—Proboscis of the Whelk.—Tongue of the Limpet.—Stomach of the Bulla, the Scyllæa, and the Sea-Hare.—Organs of Sense in the Gasteropods.—Their Caution.—Their Enemies.—Their Defences.—Their Use to Man.—Shell-Cameos.—The Pteropods.—Their Organisation and Mode of Life.—The Butterflies of the Ocean.—The Lamellibranchiate Acephala.—Their Organisation.—Siphons.—The Pholades.—Foot of the Lamellibranchiates.—The Razor-Shells.—The Byssus of the Pinnæ.—Defences of the Bivalves.—Their Enemies.—The common Mussel.—Mussel Gardens.—The Oyster.—Oyster Parks.—Oyster Rearing in the Lago di Fusaro.—Formation of new Oyster Banks.—Pearl-fishing in Ceylon.—How are Pearls formed?—The Tridacna gigas.—The Teredo navalis.—The Brachiopods.—The Terebratulæ.—The Polyzoa.—The Sea-Mats.—The Escharæ.—The Lepraliæ.—Bird's Head Processes.—The Tunicata.—The Sea-Squirts.—The Chelyosoma.—The Botrylli.—The Pyrosomes.—The Salpæ.—Interesting Points in the Organisation of the Tunicata.
Simple or compound, free or sessile, peopling the high seas or lining the shores, the marine Molluscs, branching out into more than ten thousand species, extend their reign as far as the waves of ocean roll. Though distinguished from all other sea-animals by the common character of a soft unarticulated body, possessing a complicated digestive apparatus, and covered by a flexible skin or mantle, under or over which a calcareous shell is generally formed by secretion, yet their habits are as various as their forms. Some dart rapidly through the waters, others creep slowly along, or are firmly bound to the rock; in some the senses are as highly developed as in the fishes, in others they are confined to the narrow perceptions of the polyp. Many are individually so small as to escape the naked eye, others of a size so formidable as to entitle them to rank among the giants of the sea; some are perfectly harmless and unarmed, others fully equipped for active warfare. It is evident that creatures so variously gifted, and consequently so widely dissimilar in structure, cannot possibly be grouped together in one description, and that each of the four orders, Cephalopoda, Gasteropoda, Pteropoda, and Acephala (Lamellibranchiates, Brachiopods, Polyzoa, and Tunicata), into which they have been subdivided, must be separately brought before the reader, in order to give him a clear and faithful picture of their organisation and mode of life.