The nucleus is a membrane-bound organelle that contains the cell's genetic material (DNA). It plays a crucial role in regulating cellular activities, including:

* DNA replication: The process of copying DNA before cell division.

* Transcription: The process of converting DNA into RNA.

* RNA processing: The modification of RNA before it is translated into proteins.

* Cell division: The nucleus divides during mitosis and meiosis, ensuring that each daughter cell receives a complete set of chromosomes.

So, even though mollusks are invertebrates and have relatively simple body plans, their cells still require a nucleus to function properly.