1: oral tentacle. 2: channel linking cerata. 3: buccal mass. 4: stomach. 5: ovotestis. 6: anal tract. 7: tip of foot.
Internal organs can sometimes be seen in gastropods with semi-transparent bodies.

Anatomy of marine gastropods without dissection. Ian F. Smith
Revision (2016) of article in Mollusc World 28: 13 to 15, March 2012, Conch. Soc. GB & Ireland.
PDF version available for download at www.researchgate.net/publication/310467378_Anatomy_of_mar…
Glossary below

Much anatomy can be observed on molluscs lacking external shells 1GA 1 Gastropod anatomy. Side view of Berthella plumula. , and internal organs can sometimes be seen in those with semi-transparent bodies 2GA 2 Gastropod anatomy. Transparent sole of Janolus hyalinus on undersurface of water. .
But the shell of a sea snail usually conceals most of the soft body, and only parts of the foot, head and siphon are seen when the animal is active 3GA flic.kr/p/P9HhqF .
Improved views of the sole of the foot and underside of the head can be obtained when small snails crawl on the undersurface of the water 4GA 4 Gastropod anatomy. Rissoa parva, 2.5mm long, crawling on undersurface of water. . Others can be observed 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 5GA 5 Gastropod anatomy. Juvenile Buccinum undatum crawling on underside of inverted petri dish. Holes for escape of trapped air. . 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.
Much more of a snail can be observed if it is induced to extend fully. The simplest method is to place the shell in water with its aperture facing up. A brief glimpse of behind the head, where the penis is usually located on males, may be had as the snail stretches out to grip the substrate and turn the shell 6GA 6 Gastropod anatomy. Unrestrained Peringia ulvae turning from lying with shell aperture facing up. Head and yellowish hooked penis exposed for a few seconds during the turn. . The form of the penis is often important in differentiating species, but the exposure may be too brief for the necessary detail to be studied. For longer examination, or photography, the turning of the snail needs to be prolonged. This can be sometimes done by holding the spire with forceps, but success varies between species and individuals. For example, Peringia ulvae may extend quickly, Littorina littorea is often slow to emerge and retreats at the slightest movement of the forceps, and Nucella lapillus may stay withdrawn for days if it has fed recently.
Other methods of restraint are needed for reluctant extenders. Small species, like P. ulvae, can be held for longer by pushing the spire into a prepared hole in the edge of a small piece of plasticine about 5mm thick. This leaves both hands free for operation of equipment, and allows one to do something else while waiting for the animal to extend. The effort exerted by the animal against the restraint produces an exposure of much more of the body than during a normal turn 7GA 7 Gastropod anatomy. Male Peringia ulvae restrained by plasticine. Large penis visible protruding from shell behind head. . Many can shift a piece of plasticine many times their own size, so weigh it down with a piece of lead. Often, after a few minutes, the snail manages to pull itself free, so check frequently for movement. For photography, avoid brightly coloured plasticine as it may reflect unwanted colour onto the specimen.
Larger species, especially those with short spires, cannot be held by plasticine. Instead, they can be gripped in all-plastic clothes pegs (metal parts rust). To prevent the peg from floating and the snail from moving it, place a piece of lead on it 8GA 8 Gastropod anatomy. This design of clothes peg has three points where shells may be gripped, and it has no metal parts to rust. Central peg painted black to prevent colour reflection in photography. . For photography, paint coloured pegs with black bituminous paint to avoid colour reflections. As individuals of the same species often vary in their willingness to extend, it is advisable to restrain several simultaneously to increase the chance of seeing an example of each sex. Collecting eight mature specimens in the breeding season will make it virtually certain that you have males and females if the sexes are equally frequent.
With patience, well directed lighting and luck, good views or macro-photographs will be had of the head, tentacles and, within the mantle cavity, the ctenidium, anus, and penis or oviduct opening and ovipositor. When the epithelium is thin and translucent, some internal organs may also be discerned, such as the radular sac and odontophore. 9GA 9 Gastropod anatomy. Male Littorina fabalis, restrained by clothes peg (hidden by editing). . 10GA 10 Gastropod anatomy. Littorina littorea female, restrained by clothes peg. . 11GA 11 Gastropod anatomy. Littorina littorea female, restrained by clothes peg. . 12GA 12 Gastropod anatomy. Littorina littorea male, restrained by clothes peg. .
The above techniques will assist examination under a dissecting microscope or hand magnifier, but vibration or shadow movement may cause timid species to withdraw. Best results are often obtained by photographing the extended animal with a digital SLR camera and well directed flash, and subsequently viewing the magnified images on the screen of a computer.
Live examination cannot replace skilled detailed dissection of internal anatomy, but for those who lack the skill, equipment or inclination to dissect, it can provide much interest and information. For studying external anatomy, live examination sometimes has the advantage as the positions of organs on a dissected animal depend on the way the mantle cavity is opened, and they may be collapsed or contracted, even when the animal was narcotised and relaxed before killing. On live animals the organs are in more predictable positions and more likely to be expanded . Graham, (1988), writing of Nucella lapillus, states, “The head is a flat transverse ridge…Each tentacle has an eye about one third of its length up from the base”. This description may be based on collapsed dead material as all live specimens that I have examined had the eye about two thirds of the way up the fully expanded tentacle, and the head was not flat 13GA 13 Gastropod anatomy. Nucella lapillus male, restrained by clothes peg. .

References
Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. Ray Society, London.
Fretter, V. and Graham, A. 1962. British prosobranch molluscs: their functional anatomy and ecology. Ray Society, London. (Has index of species.)
Fretter, V. and Graham, A. 1994. British prosobranch molluscs: their functional anatomy and ecology. Revised edition. Ray Society, London. (Much new material but lacks index of species.)
Graham, A. 1988. Molluscs: prosobranch and pyramidellid gastropods: keys and notes for the identification of the species. Brill & Backhuys, for Linn. Soc. Lond. & Estuarine and Brackish-water Sciences Assoc. Synopses of the British Fauna (New Series) no.2. Edition 2 (662pages). Leiden. (Edition 1 of series, 1971, 112 pages, is no substitute.)
Thompson, T.E. 1976. Biology of opisthobranch molluscs 1. Ray Society, London.
Equipment Source
Ecoforce clothes pegs; use search on Ebay www.ebay.co.uk/

Glossary, including abbreviations
1GA, 2GA etc = numbers of images in Gastropod Anatomy Album.
aperture = mouth of gastropod shell; outlet for head and foot.
buccal mass = anterior of digestive system including an odontophore that supports anterior of radula, and a complex of muscles to operate them and other mouthparts. Often red or pink from myoglobin.

chemoreception = sensing of chemicals; “smell / taste”.
chitin = semitransparent flexible horny protein.
chitinous = (adj.) made of chitin.
ctenidium (pl. ctenidia) = comb-like molluscan gill; usually an axis with a row of filaments either side.

hypobranchial gland = thickened, sometimes puckered, tissue on roof of mantle cavity of many gastropods. Secretes mucous to trap and consolidate particles from inhalent water before expulsion in exhalent current. Often other biologically active compounds produced. Gland occurs also in some bivalves and cephalopods (ink sac).

mantle = sheet of tissue covering visceral mass of molluscs. Secretes shell of shelled species, and forms part or all of dorsal body surface (notum) of those without shells.

mantle skirt = extension on gastropods of mantle proper as a flap roofing a cavity containing gills, genital and renal openings, anus etc.

myoglobin = red oxygen-binding protein in muscle tissue; often in buccal-mass muscles of gastropods. Similar to red haemoglobin in vertebrate blood, but green haemocyanin is usual oxygen-carrier in mollusc blood. See www.researchgate.net/publication/251227038_Radular_myoglo…

odontophore = firm, approximately ellipsoid, structure of cartilage supporting radula. Protruded like a tongue to operate radula. Usually reddish from myoglobin, and medially grooved.

opercular disc = part of foot that growing operculum rests on.
opercular lobe = extension of opercular disc beyond edge of operculum.
operculum = plate of horny conchiolin, rarely calcareous, used to close shell aperture.
ovotestis = hermaphrodite organ serving as both ovary and testis.
oviduct = internal tube to carry ova from ovary to the exterior.
ovipositor = structure used in depositing spawn.
radula = ribbon of chitin bearing chitinous teeth (sometimes mineralized) that is extruded on a tongue-like odontophore of cartilage to rasp food. en.wikipedia.org/wiki/Radula

radular sac = tube, ending in a caecum, where radula is created and stored.
rhinophore = chemo-receptor tentacle; nudibranch sea slugs have a pair on top of head.
siphon = extension of mantle to form a channel for respiratory water current.
siphonal canal = grooved or tubular extension of outer lip of the shell aperture on some snails, to support the siphon.
veil = flat anterior extension of head.