Though Aristotle might have observed this ribbon, it was officially discovered by Swammerdam in a work published after his 1680 death. By the mid-1700s, the ribbon was described in several species: rows and rows of tiny teeth pointing toward the stomach. The first drawing appears in a work by Poli in 1791.
Of course, these days, radula are photographed under the microscope.
Are there any differences in radula among the species? Yes. Great, then, the pattern of radular teeth becomes a diagnostic tool to identify species. (but stay tuned)
In the multitudinous work on radula (I jest) one step was the determination of the chemical composition. The first reports noted that the radular teeth were made of silica. Later reports suggested they were chitin, with others weighing in, suggesting even iron and calcium phosphate.
In early studies, the dilemma was that determining the chemical composition was done by the process of elimination. For example, silica was known to resist dissolving in acetic acid but does dissolve in hydrofluoric acid. Thus, the teeth resisted one and were dissolved by the other, at least presumably since no experimental details were given, then they might probably, kind-of-sorta, be made of silica
It turns out that the radular teeth are not made of silica
The radula is chitin, sometimes hardened by iron and calcium, but these substances are sometimes not well-crystallized. Thus, these elements do not form the lattice/shape common to that element (for example, iron forms a cube). In one study 1000 radula were extracted for analysis. One thousand individuals were dissected to determine the chemical nature of the radula, and the results ended up as a few lines in a paper. Scientists are serious people. Later analyses showed traces of other elements in some species: calcium, potassium, phosphorus, sulfur, silicon, and even copper. FYI, chitin is a carbohydrate polysaccharide that also contains nitrogen. Chitin is found in lobster and crab carapaces and forms the outer covering of mushrooms.
Many snails scrap algae or periphyton from rocks, exposing their radula to harsh abrasion. The radula is made in and emerges from the radular sac. The new teeth are added to the back of the ribbon. The front of the ribbon is the working end. Two hypothesized strategies for radula persistence are 1. make lots of teeth quickly, keeping up with the ones at the working end being worn out, or 2. Make them slow and harder/tougher, able to withstand more abrasion before being worn out. This may be the difference between, for example, the amount of iron incorporated into the ribbon.
Earlier, I stated that species determination might be aided by examination of the radular teeth: It turns out that many things about snails, the shape and thickness of the shell for example, are modified by the environment, called ecophenotypes. Thus snail species A living in a fast flowing river, may have a dramatically different shell than snail species A that lives in a shallow calm pond. Is there variation in radula in this way. The first bits I have heard of this suggest yes. Though not to the extent of something like shell shape. Pretty cool, also pretty frustrating if the radula is being used for species determination.
Though the radula is confined to mollusks, snails are not the only mollusks with radula. The deep-sea Monoplacophorans, Chitons (Polypalcophora), and Cephalopods all have radula. Indeed, the only molluscan class that does not have radula are the bivalves.
Aristotle. 1476. De Animalibus Historia.
In 1476, printers Johann de Colonia and Johannes Manthen of Venice issued Aristotle's De Animalibus, translated from Greek by humanist Theodorus Gaza. Aristotle lived 384–322 BCE.
Brütt J-O, Gorb SN, Krings W. 2022. The ontogeny of elements: distinct ontogenetic patterns in the radular tooth mineralization of gastropods. The Science of Nature 109: 58.
Call RE, and Beecher CE. 1886. Description of new Rissoi Mollusk. Bulletin of the Washburn College Laboratory of Natural History 7: 190-192. (Figure 3 image from here).
Call RE. 1886. On the genus Campeloma, Rafinesque, with a revision of the species, recent and fossil. Bulletin of the Washburn College Laboratory of Natural History 1: 149-168.
Hou J, Aydemir BE, Dumanli AG. 2021 understanding the structural diversity of chitins as a versatile biomaterial. Philosophical transactions of the royal society A379: 20200331. https://doi.org/10.1098/rsta.2020.033
de Lacerda LEM, Richau CS, Amaral CRL, de Silva DA, Carvalho EF, dos Santos SB. Ferrissia fragilis (Tryon, 1863): a freshwater snail cryptic invader in Brazil revealed by morphological and molecular data. Aquatic Invasions 10(2): 157–168. Radula photo.
Poli Giuseppe Saverio, and S. (Stefano) Delle Chiaie. 1791. Testacea utriusque siciliae eorumque historia et anatome tabulis aeneis illustrata. Parmae : Ex regio typographeio.
Sollas IBJ. 1907. The molluscan radula: its chemical composition and some points in its development. Journal of Cell Science s2-51(201): 115–136.
Swammerdam J. 1737. Biblia Naturae,' Leyden, 1737. Or maybe Bybel der natuure. Published 50+ years after his death.
Further note on Poli 1791.
Numerous illustrators and engravers signed the leaves of plates in this book. Some of those named include: (illustrators) Jos. Cammarano; Dom. Casanova; Cava; Pietro la Vega; Aniel Lamberti; Jos. Lo Manto; Franc. Morelli; Navarra; A. Siesto; (engravers) Biondi; Ioh. Brun; Dom. Casanova; C. Cataneo; Nic. Cesarano; Mo. di Pietro; Philip. Imperato ; Joan. Ottaviani; Ant. Ricciani; Rossi; Vinc. Scarpati; Angelo Testa; Toro; and Ant. Zaballi.