CHAPTER 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 Cyelostoma.--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 270