They range in size from 10 millimeters to 20 meters for the giant squid. They have a head with cephalic tentacles and a well-developed foot used in crawling. Some gastropods have lungs for respiration, others gills. This brings their organs from a posterior position to an anterior position behind their head. In most cases, the soft animal is able to retract into their shells for protection.
Gastropods are very successful in marine, freshwater and terrestrial habitats.
Their size is from 0.5 to 750 millimeters. The mantle cavity is enlarged, enclosing the visceral mass and other internal organs. There is no differentiated head or cephalic region, and the radula common to all other molluscs is absent. Most are filter feeders, with the gill acting as a food collecting and sorting organ, in addition to filling its respiratory function. The mouth usually has a pair of labial palps on either side that handle and direct the food collected by their gills.