Theodoxus fluviatilis is a freshwater snail living in rivers and brackish waters in Europe. It belong to nerites, the family Neritidae.
Malacological books usually depict Theodoxus fluviatilis like a shell only. The apertural view:
The width of this shell is 8 mm and the height of the shell is 6 mm. Shells are sometimes corroded. The lateral view:
There are remnants of two egg capsules on the shell. Snails lays eggs on solid things like stones, empty shells and shells of live Theodoxus. It is useful because such eggs are much more guarded than other eggs on non-living things.
The shell can be closed by the calcareous operculum (this operculum belong to the above shell). The length of the operculum is 4 mm and the width of the operculum is 3 mm including the rib. Outer side of the operculum:
Inner side of the operculum shows the rib and some depression near the rib.
Another shell of Theodoxus fluviatilis: the width of this shell is 8.5 mm and the height of the shell is 6 mm. There are also two remnants of egg capsules.
Live nerite snails are not easy to watch, because they are tightly attached to stones and they usually stretch tentacles and eyes only. The width of the shell is 7 mm and the height of the shell is 5 mm.
When the snail is not crawling, its foot is rounded as a suction cup:
Another Theodoxus fluviatilis snail with a different pattern on the shell. Such variability is typical for nerites and these variable patterns developed for crypsis (crypsis is a type of camouflage). The width of the shell is 7.5 mm and the height of the shell is 5.5 mm.
Right side view:
Two left side views:
Ventral view of crawling snail (there is a bubble):
Species in the Neritidae family have one gill (ctenidium) for breathing on the left side of the body. It is in the mantle cavity. The gill is called bipectinate, that means that it have two margins toothed like a comb. I was lucky to get one photo of a gill of a live snail though the partly translucent thick shell:
Detail cropped from the previous photo:
Barroso C. X., Matthews-Cascon H. & Simone L. R. L. 2012: Anatomy of Neritina zebra from Guyana and Brazil (Mollusca: Gastropoda: Neritidae). Journal of Conchology 41(1): 49-64.
(I consider this reference useful for the terminology of anatomy of nerites.)