What’s the Difference Between Snails and Slugs?

Ever just step outside after it rains and see slugs everywhere? Maybe you saw both slugs and snails coming out after watering your garden or something. Either way you probably didn’t want to touch either of them, but you might have wondered if a slug is just a snail that lost its shell. Or is a snail just a slug with a shell? What’s the difference between snails and slugs?

Gastropods

The taxonomic class gastropoda is a pretty broad banner that encompasses both snails and slugs. It includes saltwater, freshwater, and land varieties–because remember sea slugs and sea snails are a thing. For those who remember their taxonomy, you probably remember that class is pretty broad, coming after only the organism’s kingdom and phylum. The broadness of gastropods is seconded probably only by insects, and we can estimate just how broad that is by looking at molluscs. Mollusca is the phylum that contains gastropods, and probably contains like 85,000 different, named species. The upper boundary is 120,000 and the lower boundary is about 50,000. 

Because gastropods are so diverse, making normative judgements on gastropods is actually kind of difficult. It’s so difficult that the taxonomic classification for gastropods is under revision based on their genetics. Though the land ones are known for having four tentacles on their heads containing their sensory organs. They’re also known for using love darts as a means of reproduction, wherein they shoot little chitinous darts into each other as a part of their courtship ritual.

Here we thought online dating was hard.

Snails or Slugs First?

Luckily there isn’t much of an argument here like you might want to have with chickens and eggs. Snails came first. The first gastropods were all marine and came onto the land with shells. For various reasons, gastropod species started evolving to lose the shell–something that has come about independently multiple times. 

Lots of slugs actually still (kind of) have a vestigial shell, though they’re never big enough for the slug to actually retract its organs into the shell (otherwise we’d be back to talking about snails). But most vestigial shells are actually internal, and kept inside the slug’s body–the internal shell often contains digestive organs.

Land snails and slugs are actually, on the outside, basically the same aside from the shell. Slugs do need more wet habitats (very generally) than snails, since their shells make them more prone to desiccation. To support a calcium-rich shell, snails need to be in spaces where they can eat more calcium. Other than that, snails are typically a little longer (15 inches rather than 10). Slugs also have different defense mechanisms, preferring to either curl up into a ball and anchor themselves to a surface or make themselves slippery and hard to grip. Snails just get to chill out in their shells.


Look at some other snail animals here.

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