Slugs vs Snails
Belonging in the Class: Gastropoda of Phylum; Mollusca, slugs and snails are mostly different in their external structure and appearance. They could vary in size from microscopic to large and found in both land and aquatic ecosystems. With a great diversity there are more than 60,000 living species of slugs and snails in the world today.
Slugs
Basically, slugs are gastropods without shells. However, there may be either an external vestigial shell or an internal shell present in some species. Slugs have been evolved from snails but, they are evolutionarily different among themselves. There are about 60 species those are most commonly inhabit sea and fresh waters. Those who live in land are more susceptible for desiccation hence, they inhabit damp or moist environments more often than not. Slugs have two pairs of tentacles those responsible for smell and vision. The muscular foot has more mucus secreting cells to prevent the foot being irritated from walking over rough land, and as a result, a trace of their route is left after they have walked. Rhythmic muscular movements in the foot cause them to move and overall speed of walking is very low, causing sluggish movements. They eat dead leaves and fungus, making them detritusvores. Slugs have many natural enemies viz. amphibians, reptiles, birds, mammals, and fish. Their slimy body acts as an adaptation against the predators. One of the most interesting behaviors in slugs is the apophallation, in which they bite off the penis of the partner after mating if it is stuck inside after transferring sperms. The penis is locked inside as a result of it being very complicated in structure and will make sure that no one else will mate with her. Slugs are, however, harmless animals but some species are notorious as pests on agricultural crops.
Snails
Shelled gastropods are called snails, and they are very highly diversified with more than 2500 species in the world. The body of the snails is coiled and covered with a similarly coiled shell. Size, colour, and shapes vary drastically among species and those characters are useful in identification for the taxonomists. Mucus secretion prevents any abrasion from walking but, traces of routes are not common. There are comparatively more marine species and some freshwater species with few species of snails on land. Either lungs or gills are present for respiration. Snails could retreat into their shell when they are alarmed. Their tooth-like structures called radula are used in grazing but, many marine species are predatory and omnivorous. People in Mediterranean, Southeast Asian, and European countries prepare delicious dishes of snails onto their dining tables. Many land snail species are harmless but, some are serious pests.
What is the difference between Slugs and Snails?
Both belong in the same taxonomical class sharing many similar features viz. muscular foot, coiled body structure, chemosensory and light sensory tentacles, mucus secretion, presence of both sexual organs in the same animal… etc. However, the slugs secrete more mucus in comparison to snails. Land slugs are more susceptible for desiccation than land snails. Diversity is very high among snails compared to slugs. As the most important distinction between them, the external shell is prominent in snails but absent in slugs. The interesting behaviour of apophallation in slugs makes them unique in the Kingdom: Animalia.