Over exploitation, pollution and habitat degradation are the principle threats to Gastropoda. Others include introduction of species such as the goat to the Galapagos islands which have degraded the vrgetation causing erosion and subsequently the loss of land snail habitat.
Also known as pelecypods, bivalves include the familiar clams, oysters, and scallops. They are defined by the presence of two laterally-compressed shells, hinged together by an elastic ligament and shell teeth. Most adult bivalves are sedentary filter feeders, using currents set up by cilia on their gills to bring in food particles. Particles are trapped in mucus secreted by the gills and labial palps, and moved to the mouth by ciliary action. A few bivalves, such as scallops, are able to move rapidly by clapping their valves together. Others are able to move slowly by muscular action of their foot. Still others are completely sessile, attaching permanently to the substrate by means of secreted byssal threads.