Organisms whose cells have no visible nucleus are called prokaryotes.

Here's why:

* Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and other membrane-bound organelles.

* Their genetic material (DNA) is located in a region called the nucleoid, which is not enclosed by a membrane.

* Examples of prokaryotes include bacteria and archaea.

In contrast, eukaryotes are organisms whose cells have a true nucleus and other membrane-bound organelles.