Molluscs

Maria and Sofia are in the garden looking at the plants. Some of the plants have holes in their leaves; snails have moved in. Here they are, and there’s some slugs as well! Apart from that snails have a shell and slugs don’t, the two animals look quite similar. Both types of animals have a long, soft, and slimy body.

This is made up of two parts: the head, and a strong muscle which is used for moving around: the foot. Slugs and snails have two pairs of tentacles on their heads: the antennae. One pair is used for sensing and smelling, and the other one has eyes at the tips. Snails and slugs belong to the same class of organisms, called: gastropods. They are the biggest class of a larger group of organisms, a phylum, called molluscs!

Clams, mussels, and oysters are examples of animals that belong to the second largest class of molluscs. We call this class bivalves. Their most noticeable characteristic is their two-part shell. The shell protects the soft body of these animals. The two parts of the shell are joined by strong tissue.

The bivalves don’t have a head, but they do have a strong muscular foot, similar to that of gastropods. They use it to move around, to attach themselves to a surface, or to burrow into the bottom of a lake, or the sea. Members of the third class of molluscs, such as octopuses, squid, and cuttlefish, only live in the sea. The name of this class is cephalopods. Animals in this class have the most complex body forms, and the most developed nervous systems.

Their head and foot are completely merged, and there is a ring of limbs around this part of the body. They use these limbs for feeding, moving, and defending themselves. Different groups of molluscs look very different, and they all have their unique characteristics. But there are a few things that are typical for the majority of animals in this phylum. One of the common characteristics is the outer layer of skin tissue that all molluscs have on the back.

This tissue forms an outer body layer called the mantle. The mantle is also the body part that produces the hard shell. Some molluscs, like snails and clams, have hard outer shells, but some others, like the cuttlefish, have small internal shells. Molluscs are also the only animals that have a special mouth part called a radula. The radula looks a bit like a tongue, and is covered in small teeth: it’s perfect for cutting and chewing food!

Bivalves are the only molluscs that don’t have a radula, instead they eat by filtering the water they live in. Some species can filter several tens of litres of water every day! By filtering water, they clean it of potentially harmful substances, like nitrogen and phosphorus. This makes them a very important part of aquatic ecosystems. Other molluscs are really important too.

They are food for other organisms, and also for us humans. But molluscs like snails can, if there are too many, cause damage to plants. It’s good that there are only a few of them in Maria and Sofia’s garden.