Mollusca is the animal kingdom's second-largest phylum, accounting for a significant portion of the world's fauna. The Gastropoda are the only molluscs to have successfully inhabited land. Gastropoda is a large part of the phylum Mollusca and the most diverse class in the phylum. They consist of about 65,000 to 80,000 species. Gastropod structure, behaviour, feeding, and reproductive adaptations differ greatly from one group to the next. When the term "snail" is used in this broad sense, it refers not only to land snails but also to a variety of sea and freshwater snail species. Slugs are also gastropods that don't have a shell or have just an internal shell. On the other hand, snails have a hard shell and this is what differentiates them from a slug. Gastropods Inhabitat various places like woodland, gardens, mudflats, mountains, sandy subtidal, estuaries, lakes, hydrothermal vents and many more. Some of them also have parasitic features.
Types of Snails
Achatina Fulica – A Giant African Native
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Achatina Fulica is a giant native of Africa. It is the largest of the species with a length of about 20cm. Because of its high reproductive rate and voracious appetite for crops and vegetation, it is considered an invasive species in some areas.
Helix Aspersa – Found in Gardens
These are small snails that are known as the fastest ones and are usually found in gardens. They are native to the Mediterranean, Western Europe, Asia, and northern Egypt. It grows up to 1.3 inches and has a unique shell structure that sets it apart from others. It has a maximum speed of 0.047 km/h.
Helix Pomatia – Roman Species
The shell of this one is so exquisite that it accounts for almost a third of its total weight and it was only found in Europe, but now it can be found all over the world. It can survive in temperate forests with humid climates and no rainfall.
Snail animals can be found all over the world, so it's no wonder that they can live in a variety of environments. Some prefer to live in the desert, while others prefer to live in ditches and colder climates. They are also found in mountain ranges and even in marshes. Other snails have evolved to live in ditches, near deepwater hydrothermal vents, the crashing waves of rocky coasts, caves, and in several other environments.
There is a saying about snails and slugs, that is, these species cannot dwell in the desert, but it is not true. Not only desert, but terrestrial gastropods can survive in any environmental condition including mountainous regions, high altitude regions, and even in and hot and cold climatic conditions.
They have a thin translucent shell that is entirely made of the protein conchiolin and can still be found in environments where there isn't enough calcium carbonate to create a truly strong shell, such as certain acidic soils on land.
Food and Feeding Habits
Snails, like all other molluscan classes except bivalves, have a firm odontophore at the front end of the digestive tract. This organ usually supports a long ribbon (radula) with a few thousand to several thousands of “teeth” on it (denticles). Food particles and debris reach the oesophagus as muscles extrude the radula from the mouth, spread it out, and then slide it over the supporting odontophore. The most primitive form of gastropod feeding appears to have involved collecting and grazing algae from rocks. The basic rhipidoglossan radula, with many slender marginal teeth arranged in transverse rows, is still present in some Archaeogastropoda species.
The base structure of the body of an animal is made up of the skeletal system, which supports the whole system. Invertebrates have a skeleton that is either internal or externally rigid but non-bony. The type of skeleton an animal has is a distinguishing feature of the phylum to which it belongs.
Snail animals are in the Mollusca phylum, which is the largest invertebrate animal phylum.
Their behaviour, ecosystem, and systemic organization all show diversity.
Snail’s Body is Divided into Three Parts:
They also have a hard outer covering on their back that serves as the skeleton. The shell protects its internal parts from extreme climatic conditions. The external skeleton, also known as an exoskeleton, is the shell on its back. For muscle attachment, they have a wide surface. These often serve as camouflage and protection from predators and mechanical stress. While every gastropod has a shell, it has reduced and facilitated slug mobility during embryonic development.
The shells of snail animals are usually made up of three distinct layers, the majority of which were calcium and about 2% protein. These shells are non-cellular structures, unlike animal body structures. Mantle tissue is a part of a mollusc’s body that is found within the shell. The tissues of the mantle come into close contact with the shell. For the formation of the shell, they secrete and precipitate minerals such as calcium and proteins. Proteins serve as the shell's building blocks, while calcium polishes it.
Since the exoskeleton does not shed, the shell grows from the bottom to the top as the body grows. The three layers of the snail animal's shell are the inner pearly layer of nacre, the prismatic layer, and the proteinaceous periosteum. The inner and middle layers are calcium-rich and calcified, while the outer layer is protein-rich and uncalcified. The shells can be of various shapes and sizes. Shells are usually spirally coiled, but some species, such as limpets, have cone-shaped shells.
The head is typically symmetrical on both sides, with one or two pairs of tentacles, sometimes with accessory palps, and the mouth in the centre of the ventral margin. The upper tentacles, or ommatophores, of Stylommatophora land snails, are invaginable, and the eyes are carried at the tips. The eyes are located at the base of the tentacles in freshwater basommatophorans and most prosobranchs, but in some forms, such as Strombus, the eyes are raised onto an accessory stalk. Tentacles on prosobranchs will contract. The lateral lips of carnivorous gastropods from lobes are called labial palps, which aid in the detection of prey. The mouth is sometimes extended into a proboscis that reaches far ahead of the tentacles. Carnivorous animals frequently have a proboscis that can reach great lengths and is either invaginable or contractile.
The "foot" of a snail is a muscle that enables it to travel slowly across the ground. Although the foot is a flat, narrowly tapered, muscular organ that is highly glandularized and typically ciliated in its basic form, it undergoes numerous modifications in different classes. A propodium and a metapodium are often divided anteriorly and posteriorly, with the former capable of reflexing over the shell. The foot of the Strombus is very narrow; in limpets and abalones, it is very long and acts as an adhesive disc. The foot is a swimming organ in pelagic gastropods, especially heteropods and pteropods. Parapodia are lateral projections of the foot found on many prosobranchs and some opisthobranchs that are used for swimming or are reflexed over the shell surface. The tendency to self-amputate the posterior portion of the foot, which remains wriggling vigorously to confuse a predator while the anterior foot and visceral mass crawl slowly away to safety, is present in many types of land slugs, several nudibranchs, and the neogastropod marine family Harpidae.
In inland gastropods, the foot is the locomotion organ. The foot, on the other hand, is significantly diminished or changed in swimming and sessile forms. A snail's normal progression is governed by muscular contraction waves that travel from the posterior to the anterior end of the gliding portion of the foot. The foot is divided into right and left parts in a few groups, with different waves passing on either side.
The majority of prosobranchs move slowly, at less than eight centimetres per minute, though Haliotis has been recorded to move at nearly ten times that pace. Many opisthobranchs move by using their foot musculature, but others use ciliary motion to glide on the underside of water-surface films.
For locomotion, land pulmonates depend on a combination of muscular action and cilia. The foot is divided longitudinally into three sections in many of these animals, with locomotor operation restricted to the central segment, which glides on a mucous track.
Reproduction in Snails
Although most snail animals have male and female sexes, terrestrial pulmonate snails (Stylommatophora) are both male and female hermaphrodites. Some freshwater snails, as well as the marine opisthobranchs (Opisthobranchia), make up the vast majority population.
Male and female organs, as well as hermaphroditic organs, are found in one typical genital apparatus in hermaphroditic ones.
In terrestrial species, mating can be a fascinating sight to see. Leopard slugs, for example, mate when suspended in mid-air on a slime cord. Roman snails, on the other hand, mate on the surface, but the process can take several hours. A love dart can also be used during the procedure. Copulation occurs simultaneously after hermaphroditic Romans copulate, and vice versa.
After fertilization and oviposition, the embryonic development of the young takes place in the egg in the terrestrial ones. Complete young organisms hatch from the egg, which must develop but also have a complete set of organs and a shell. Some marine gastropods, on the other hand, reproduce through a series of larval stages (Trochophora and Veliger larvae), which swim freely or float in the water as plankton.
Amazing Facts about Snails
These species are generally regarded with contempt. Horticulturists are especially hostile to their slimy neighbours, and they will go to any extent to get rid of them.
However, there is a lot of fascinating snail facts to share, but only a few people know them. For example, they can crawl over a knife's edge without getting hurt. They can even cross water - or, at the very least, crawl along its surface. Some of them consist of venom that uses a harpoon tooth to destroy their prey and others that surround a mussel before it opens its shell. Slugs that live in the sea eat jellyfish only to collect their nettle cells and use them for self-defence.