How do sponges differ from flagellate protists

Sponges have specialized cells that resemble the parts of the same animal unlike a group of individual cells composed of individual organisms. Protists are unicellular, and live in isolation even when there's a bunch of them stacked close to each other. They are unspecialized cells.

Sponges have a body skeleton that is not composed of cells. They perform sexual reproduction by forming both the male and the female gametes. All these features are absent in the protists.

the answer to your question is sperm

the correct answer is:

a population changes in response to a change in the environment.