The Gastropods, belonging to the Mollusca phylum is the largest group of animals, showing a wide variety of species, varying in habits, forms as well as habitats. This group is the largest of the mollusks. It consists of around 62,000 species, living at present. About 80 % of those species comprises of Molluscs. The actual species in this group extends to more than 1, 50,000. These species have been distributed in13, 000 genera and around 611 families, out of which, 202 species have gone extinct and can be seen only in the records of the fossils. The fossil record shows that, the Gastropod has been present since the times of the Early Cambrian era. This group is more commonly known as slugs or snails.
The Gastropod of one clad is very different in many aspects from the other clads, especially in the terms of habitats. Some gastropods are found in woodland, garden, rocky intertidal, small ditches, flat areas covered with mud, large rivers and lakes and sometimes on the mountains. It is the only group of animals that has invaded the land. The other synonyms of Gastropods are Univalves and Gastropoda.
The Gastropod has around 65,000 different species with different habitats. Out of these, around 5000 species are found in fresh water, 30,000 species reside on land and the other 30,000 are marine animals. Thus, Oceanic gastropods display a wide variety of species, found in diverse shell structures in the water bodies. Around 700 species of gastropods can be found from the habitat of coral reefs. But in the reefs, each species is represented by a very small number of organisms, whereas in the Arctic coast or in the subarctic area, few species with many individuals are found.
Snails residing in the ponds, lakes, marshes and streams usually represent few species but in a large number. A large number of species were produced by the complex and extensive snail radiation, that occurred in the recent geological period, particularly in old river and lake habitat like Ohrid in Macedonia, South America's Titicaca, and Southeast Asia's Mekong basin.
Size range and diversity of structure
There is a huge diversity in the structure and size of each species found in the Gastropod group. Usually, the forest-litter and marine snails grow about 1 millimeter in diameter, even after reaching maturity, whereas the African Achantin, which is titled as the largest snail found on the land, develops a shell of about 20 cm in length. The Pomacea, the largest snail found in the fresh waters of South America grows approximately 10 cm or 4 inches in diameter. The largest snail found in the marine habitat, called as the Australian Syrinx Aruanus grows up to 0.6 meters, and very occasionally more than that. Most snails are seen in smaller size; around 90% of the snail species grow only about one inch in diameter.
The gastropod features a very diverse anatomy in different species. The most sensory part in the body of the Gastropod is the head, which develops tentacles, shaped like horns. These tentacles are situated above the mouth, and the mouth is located in the base part of the head. The mouth consists of a ribbon like structure known as Radula, which helps in the feeding process along with enormous teeth growing on it. In most of the species, two eyes are situated near the base part of the tentacles. The foot is, generally a muscular and tough structure with a flat base which helps it to move with undulation. The shells are hollow and twisted towards the end.
Most species of the Gastropod are separate sexes, or can be called as dioecious. But the species living on the land are hermaphrodites. There is a great diversity in the reproductive system of each species of the Gastropod group. In the most primitive species, found in the water habitat, gametes were released into the water. Thus, fertilization took place outside the body of the female gastropods. Later, after the fertilization of the eggs, it turns into a free flowing structure or Trochophore larva. The Ciliary girdle of the larva expands into large ciliated lobes or vela. This causes the larva, also referred as Veliger, to undergo the process of torsion in which, the body of the larva twists in a 180 degree posture. This leads to the positioning of the posterior part of the larva to the anterior part that is just behind the head. The process of torsion is uniquely carried out in the gastropods.
In the Gastropods, at the end of the digestive tract in the anterior part, there is a strong odontophore which supports the Radula (ribbon) with denticles to carry the food. The radula consists of few or sometimes nearly thousand of denticles like teeth. The radula slides over the supporting organ and carries the food particles towards the esophagus. The number of denticles varies with each species, depending on the nature of feeding and food habits. Most of the gastropods are herbivores, scavengers, predatory carnivores and parasites. Some of them lack radula and they are the Ciliary feeders. Many marine species are found grazing and browsing on algae that grows on the rock, whereas the land dwellers chew fruits, leaves and barks of the tree. They also feed on the decomposed animal remains. Some of the sea slugs are carnivores and a few are herbivores. Some of the carnivorous Gastropods are Testacella, Cone shells, Ghost slug and Daudebardia.
As in the living gastropods, the fossils too, have a varied variety of shapes, colors and structures. The earliest Gastropod belongs to the late Cambrian era, which means it is about 500 million years old. There is confusion over the small shelled fossil known as Aldanella, found in the late Cambrian rocks. It is considered as the oldest form of Gastropod by some of the researchers, whereas some say it is a form of a worm. Hence paleontologists believe that the Gastropod has emerged much before the Cambrian period. By the end of this era, Gastropod developed into many varieties and was abundantly found. The earliest found Gastropods were marine animals. It is said that about 248 million years before, during the Mesozoic era, many species from the marine habitat adapted to the freshwater and land environment. The marine snail fossils are found in abundance from the Permian and Pennsylvanian rock in the eastern part of the Kansas and also in the Cretaceous rocks found in the western region.