The gastropods are animals like snails and slugs, limpets and whelks, abalone and cone shells. They form the largest class in the Phylum Mollusca. Gastropod means stomach-foot and all the members of this class are possessed of this strange characteristic: a big muscular foot on which the body is set. Most gastropods have a protective shell, which is secreted by the mantle enveloping the animal’s fleshy bits, but although this shell is a safety feature, it is also bulky and hard to move about, so some gastropods, the slugs, have done away with it.
The strangest characteristic of shelled gastropods is that they are twisted. Meglitsch (p 297) puts it this way: “Gastropods have two characteristics which would disqualify most animals from successful competition. Most gastropods live with the body coiled up in a spiral. As if this were not enough, while they are embryonic the visceral mass is suddenly twisted about, bringing the back end up to the front. Most of them remain permanently twisted and while a few partially untwist during later growth, all show its results. These two features of gastropod structure can never be ignored, for they explain a great deal about the course of gastropod evolution and enter into the organization of the organ systems.”
The problem of living in a shell that has to grow as the animal grows led to the solution of twisting. To quote Meglitsch again (p298): “”As soon as gastropods began to grow conical spiral shells, they entered into a completely new and complex biological maze. Right and left sides must grow unequally and in such a manner that a compact and manageable spiral results, if they are not to be handicapped by the shell. The achievement of the delicately balanced growth rates of whorl diameter, and of the outward and downward growth of the whorls as they coil, required to make shells of acceptable design, has been a remarkable biological phenomenon.”
Modern gastropods are twisted but they did not start out that way. Shelled gastropods first appear in the fossil record in the Cambrian period. The now extinct order Cynostraca consisted of gastropods with conical, untwisted shells, shaped a bit like dog teeth. Next came the coiled but untwisted gastropods of the extinct order Cochleostraca, which had spiral shells. The last of these old-fashioned molluscs were gone by the end of the Permian. Since that time their twisted and coiled descendants have spread out into almost every available habitat. They are particularly successful in the intertidal zones. Worldwide there are about 40,000 known species of these twisted little animals, from abalone to snails to limpets and whelks, cone shells and many others beloved of shell collectors throughout the centuries. A few, the land snails and the slugs, have even successfully colonised terrestrial habitats.
After the appearance of the spiral shell, the next step in gastropod evolution was torsion. This occurs during a larval stage known as the veliger. The body is curved, with both head and tail showing from within the mantle. The visceral hump begins to form a spiral and the whole mass revolves 180 degrees in as little as a few minutes. The results of torsion are drastic with the esophagus now entering the stomach from behind and the gills and anus going from a posterior to an anterior position. As wierd as all this is, it must have survival value, for all living gastropods go through this twisting procedure.
One of the first problems created by torsion was the placing of the anus directly over the head which leads to sewage pollution of the respiratory and sensory organs. This has been solved by reducing the gill and kidney on the larval left/adult right hand side so that water is routed through the mantle cavity from the left, over the remaining gill and then past the anus on the way out, so that wastes are carried away from the organs.
Modern gastropods are divided into two subclasses. The first, the Prosobranchia, are gastropods which undergo torsion. Limpets, marine, freshwater and land snails, pelagic snails, periwinkels and cowries, whelks and cone shells are all prosobranchs. The other subclass, the Opisthobranchia, are marine gastropods with surface gills, a nervous system untwisted by detorsion and a strong tendency to shell reduction and a secondary return to bilateral symmetry. Sea hares are opisthobranchs as are slugs and some of the terrestrial snails.
No one who has walked in the intertidal zone or snorkelled on a coral reef can fail to appreciate the beauty and success of gastropods. Many a gardner has also cursed the success of the less beautiful land snails and slugs but they too form an important part of their ecosystems. On my subtropical property in Australia, I sometimes come across the most amazing slugs. One is small and brown until touched. Then it oozes a deep purple dye as some sort of chemical defence. Another is small and grey when young but when it matures it is as large as a human hand. It turns porcelain white and has a bright red diamond on its back, a truly beautiful slug! There are also big brown banded land snails, a leftover from Gondwana days and evidence for the long term success of the twisted but beautiful gastropods.
Reference: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press.