The cell types which lack a membrane-bound nucleus are called prokaryotes. They are found in two domains:

* Bacteria: These are single-celled organisms that are incredibly diverse and abundant in the environment. They can be found in soil, water, air, and even inside other organisms.

* Archaea: These are also single-celled organisms, but they are often found in extreme environments like hot springs, salty lakes, and deep-sea vents.

Therefore, the answer to your question is bacteria and archaea.