Small snails that do not crawl on the undersurface of water can be observed ventrally by placing them in a submerged plastic petri dish with holes drilled near the edge. When they have a grip, invert the petri dish, letting any air escape from the holes. A small weight may be needed to hold the dish down. Alternatively, use a microscope slide or, for larger specimens, a submerged sheet of glass held up on supports. Species adapted to crawling on sand, such as Tritia reticulata, often have a weak grip when on inverted glass, and adults of large species, such as Buccinum undatum, may be unable to support their weight. In such cases, try smaller specimens.
Full article of Anatomy of marine gastropods without dissection. below image 2 2 Gastropod anatomy. Transparent sole of Janolus hyalinus on undersurface of water.