MOLLUSCA (-1us'ca), in zoology ac cording to Linnmus, an order of Vermes, distinct from Testacea, which immedi ately follows it. Cuvier made the Mol lusca one of the four great "divisions" or sub-kingdoms of the Animal King dom, of equal rank with the Vertebrata, the Articulata, and the Radiata. He subdivides it into six classes: Cephalop oda, Pteropoda, Gasteropoda, Acephala, Brachiopoda, and Cirrhopoda. Except that the last class has now been merged in Crustacea, and placed with the Ar ticulata or Annulosa, the essential features of Cuvier's arrangement have still been preserved. Dr. Henry Wood ward defines the Mollusca as animal's with a soft body, without segments, naked or covered with a shell of one or two valves composed of carbonate of lime secreted by a fold of the skin—the mantle. They have a brain-mass, and foot and mantle ganglia. Some have an internal hard shell or cartilage. The symmetry of the body is bilateral. Ex ample, the cuttle-fish, the snail, the oyster, etc. He makes Tunicata and the
Molluscoida an "intermediate group," and divides the sub-kingdom into four classes: Cephalopoda, Gasteropoda, Pter opoda, and Conchifera. Many thousand recent Mollusca are known, distributed throughout every climate and nearly every part of the world.
The shells of the Mollusca being all but indestructible, and easy of identifica tion, afford us a reliable means for as certaining the relative age of strata. As some, moreover, inhabit fresh waters, others the land, besides the large num bers which find their home in salt water, they often settle the fresh-water or ma rine origin of a stratum. The marine ones being distributed also in certain zones of salt water, they frequently af ford materials for sounding a sea which has passed away ages ago. Next to the Protozoa, the oldest fossils known are Mollusca. See BIVALVES.