gasteropod a

pl. [Pref. gastero- (q. v.); Gr. pous (gen it. podos )=a foot, and Lat. neut. pl. suff.-a.] 1. Zool.: Gasteropods. The most typical, though not the most highly-organized cla of the subkingdom Mollusca. Its e ential character is that the under side of the body. constitutes a single muscular foot, on which the animal creeps or glides. Most of the Gasteropod a have univalve shells, a few have them tubular or conical; in one the shell is multi-valve, and in some it is internal or wanting. Most of the spiral shells are dextral, a few are sinistral. Some have an operculum closing the aperture of the shell. The animal has a head furnished with two, four, or six tentacles, or these are wholly wanting. There is a mantle, in the folds of which the shell is produced. Some breathe air, the others water. Cuvier divided the Gasteropod a into eight orders, (1) Pecten i branch i at a, (2) Scut i branch i at a, (3) Cyclo branch i at a, (4) Tabuli branch i at a, (5) Pulmonata, (6) Tectibranch i at a (7) Inferobranchiata, and (8) Nudibranch i at a. Woodward and others have divided the cla into four orders only. (1) Prosobranch i at a, including the first four of Cuvier's orders, (2) Pulmonata, corresponding to his 5th, (3) Opisthobranch i at a, comprehending his 6th, 7th. and 8th orders; and (4) Nucleobranchiata, which Cuvier had made a distinct cla -Heteropod a (q. v.). The Prosobranch i at a have been arranged in two divisions: Siphonostomata and Holostomata; and the Opisthobranch i at a also in two: Tectibranch i at a and Nudibranch i at a. 2. Palæont.: Gasteropod a are found in all the formations from the Upper Cam brian rocks till now. Mr. Robert Ethe ridge, Paleontologist, estimated the known Gasteropod a (excluding Pteropod a) from the Paleozoic rocks: at 96 from the Silurian, 0 from the Old Red Sandstone, 46 from the Devonian, 7 common to the Devonian and the Carboniferous, 174 from the Carboniferous beds, and 25 from the Permian. Of the Pteropod a there were 27 Silurian and 1 Devonian. The Holostomata are more abundant in the Paleozoic period, and the Siphonostomata in the Secondary and Tertiary strata. The Pulmonata, from being many of them land animals, are le fully represented: the Nudibranch i at a, from the absence of the shell, not at all. Both families of the Nucleobranchiata have fo il forms. găs-ter-op-o-do us, a. [Mod. Latin gaster of M pod(a); Eng., ., suff. -ous.] Zool.: Using the belly or the whole under-part of the belly as a foot to crawl by; of or belonging to the Gasteropod a (q. v.).