Mollusks are a group of animals. They have no internal skeleton, meaning no bones. A good example is a squid. Some mollusks have a hard shell as their external skeletons, such as mussels or some snails.
Most species live in the sea. But they are also found in lakes and rivers. The water helps them carry the body. Then he is weightless. Only smaller species live on land, such as certain snails.
The mollusks are also called “mollusks”. This comes from the Latin word for “soft”. In biology, the mollusks form their own tribe, as do the vertebrates or the arthropods. It is very difficult to count how many species of mollusks there are. Some scientists call 100,000, others less. This is because it is difficult to differentiate between the different types. For comparison: There are also around 100,000 vertebrates, while insects are probably several million.
What do mollusks have in common?
Mollusks have three body parts: the head, the foot, and the sack containing the intestines. However, the head and foot sometimes look as if they are made of one piece, for example in the case of snails. Sometimes a shell is added as a fourth part, as with the mussels.
All mollusks except mussels have a rasping tongue on their heads. It’s rough as a file. The animals grate off food with it because they have no teeth.
All mollusks have a strong muscle called a “foot”. It is best seen in the snails. You can use it to move or burrow.
The intestines lie in a visceral sac. This is a separate part of the body that is surrounded by a coat. It contains the esophagus, stomach, and intestines. There is a simple heart. However, this does not pump blood through the body, but rather a similar fluid, hemolymph. They say “hemolums”. In most mollusks, it comes from the gills, where they absorb oxygen. Only snails that live on land have lungs. The heart pumps hemolymph into the body.