Simple Animals: Investigating Mollusks

The video explains that molluscs, such as clams, squid, and mussels, are invertebrate animals. Most molluscs have hard external shells, while some, like squid, have internal shells, and others, like slugs, have no shells. Molluscs possess well-developed nervous systems, including a brain and sense organs like eyes.

Which molluscs do not possess shells at all?

What characteristics do molluscs have regarding their nervous systems?