nPolyplacophorasmall class of marine mollusks comprising the chitons; sometimes considered an order of the subclass Amphineura
Webster's Revised Unabridged Dictionary
n. plPolyplacophora(Zoöl) See Placophora.
Century Dictionary and Cyclopedia
polyplacophoraAn order of isopleurous gastropods, exhibiting bilateral symmetry and metameric segmentation. The dorsal shell is in eight successive pieces, sometimes embedded in shell-sacs; there are numerous gill-combs and olfactory tracts, or ctenidia and osphradia; paired genital ducts distinct from the paired nephridia; and there is a well-developed odontophore with numerous lingual teeth on the radula. The order is conterminous with the family Chitonidæ in a broad sense. In J. E. Gray's classification (1821) it was one of 9 orders of cryptobranchiate gastropods. The original form was Polyplaxiphora. In Gray's system of the mollusks it was considered as a suborder of heteroglossate scutibranchiate gastropods, and defined as having the gills in two lamellar series on each side of the hinder part of the under side of the mantleedge, and the shell formed of eight imbricate valves.