MOLLUS'CA (Lat. nom. p1. nen. of niollus cus, from mollis, soft). One of the chief divi sions, or phyla, of the animal kingdom, the study of which is Conchology or The body is bilaterally symmetrical except in snails (Gastropods), not segmented as in worms and arthropods, but soft, fleshy, and usually protected by a bivalve or univalve shell; moving by a 'foot' or muscular creeping disk in gastropods, or, in bivalves, by a tongue-like process; breath ing by external gills which are either lamellate or plume-like. A shell is secreted by the fleshy mantle, and in nearly all except the Pelecypoda (q.v.), which are headless, the mouth is armed with an 'odontopliore.' an apparatus of muscles and tendons hearing a rasp-like 'lingual ribbon' radnla ) for sawing or cutting the food, o• for drilling holes through shells. Many mollnsca in their young or larval swimming stages begin as a 'troc•hosphere' (q.v.), pass through a 'veliger' stage, living at the surface of the sea and gradu ally sinking to the bottom by gravity as the shell grows larger and heavier. The Mollusca form a highly specialized group, the number of species amounting to upward of 40.000, about one-half of which are living, the other extinct ; the earliest known species occur in Cambrian rocks. The
group has a wide geographical and hathymct•ical range, occurring in all seas from the shore to the abysses of the ocean and also on land and in fresh waters.
The affinities of this phylum are not yet settled. INIcdhisks have evidently descended from sonic worm-like form, as their larvae are in sonic eases segmented and like the troehosphere and `cephalida' stage of sea-worms (Annelida In the adult segmented organs, like those of worms, are often present. On the whole they are a grade inferior to the spiders and insects. Mollusks are divided into five classes, i.e.: (I) Amphineura ; (2) Pelec•ypoda (q.v.) or Lamellihranchiata ; (3) Scaphopoda (q.v.) ; (4) Gast•opoda ; and (5) Cephalopoda (q.v.). The Ainphinenra com prise a few forms formerly supposed to he worms and Neomenia). These are primi tive types from which, as smile authors think, the other mollusks may have descended; all have a ladde•-like nervous system, as in Chit on, which also has a segmented larva, mud are either shell less or somewhat like that of some TurbelIaria, (q.v.) and Pcripatus (q.v.). Sec MobLusK.