Organisms whose body temperatures change with the environment are called ectotherms.

Here's a breakdown:

* Ectotherms: They rely on external sources of heat to regulate their body temperature. This means their internal temperature fluctuates with the temperature of their surroundings.

* Examples of ectotherms: Fish, amphibians, reptiles, insects, and most invertebrates.

In contrast, endotherms are organisms that maintain a constant body temperature, regardless of the external environment. They do this through internal mechanisms like metabolism and sweating. Mammals and birds are examples of endotherms.