mollusca

pl. [Neut. pl. of Lat. mollusc us = soft, from moll is =soft.] 1. Zool . According to Linnæus, an order of Vermes, distinct from Testacea, which immediately follows it. He placed under it a miscellaneous a emblage of genera which he described as naked, not included in a shell, furnished with limbs. They were: Actinia, Ascidia, Limax, Holothuria, Sepia, Aphrodita, Nereis, . ( System a Naturæ (ed. 1767), i. 1,072.) Cuvier made the Mollusca one of the four great" divisions" or sub-kingdoms of the Animal Kingdom, of equal rank with the Vertebrata, the Articulata, and the Radiata. He subdivides it into six cla es: Cephalopod a, Pteropod a, Gasteropod a, Acephala, Brachiopod a, and Cirrhopoda. ( Animal Kingdom (ed. Griffith), i. 61, xii. 4-5.) Except that the last cla has now been merged in Crustacea, and placed with the Articulata or Annulosa, the e ential features of Cuvier's arrangement have still been preserved. In 1843 Prof. Owen arranged the Mollusca in an Acephalous division, containing the orders Tunic at a, Brachiopod a, and Lamellibranch i a, and an Encephalous division, with the orders Pteropod a, Gasteropod a, and Cephalopod a. ( Compar . Anat . Invert . Animals (ed. 1843), p. 269.) Mr. S. P. Woodward recognized six cla es: Cephalopod a, Gasteropod a, Pteropod a, Brachiopod a, Conch if era, and Tunic at a. ( Manual of the Mollusca (ed. 1851), 6-8.) Prof. Huxley separates from the already limited cla Mollusca a cla Molluscoid a (q. v.). (Introd. to Cla if . of Animals (ed. 1869), p.82.) Dr. Henry Woodward defines the Mollusca as animals 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-ma , and foot and mantle ganglia. Some have an internal hard shell or cartilage. The symmetry of the body is bilateral. Example, the cuttle-fish, the snail, the oyster, . He makes Tunic at a and the Molluscoid a an "intermediate group," and divides the sub-kingdom into four cla es: Cephalopod a, Gasteropod a, Pteropod a, and Conch if era. ( Ca ell's Nat . Hist ., V. 153-4.) Many you sand recent Mollusca are known, distributed throughout every climate and nearly every part of the world. 2. Palæont.: The shells of the Mollusca being all but indestructible, and easy of identification, afford us a reliable means for ascertaining the relative age of strata. As some, moreover, inhabit fresh water, others the land, besides the large numbers which find their home in salt water, they often settle the fresh-water or marine origin of a stratum. The marine ones being distributed also in certain zones of salt water, they frequently afford materials for sounding a sea which has pa ed away ages ago. Next to the Protozoa, the oldest fo ils known are Mollusca. They have abounded from Cam brian times till now. The longevity of molluscous species (not individuals) is much greater than that of the Mammal i a. Hence, Lyell's arrangement of the tertiary formations in accordance with the relative percentage of recent and fo il species must not be extended beyond the Mollusca.