In which organism can you find structures known as proglottids?

Proglottids are defined as segments that make up the body of certain flat animals, specifically within the class Cestoda, which includes tapeworms. These segments contain reproductive organs and are critical for the tapeworm's life cycle as they facilitate reproduction by breaking off from the main body and being excreted, often containing numerous fertilized eggs.

In the case of tapeworms, each proglottid can function independently to a certain extent, allowing them to maximize reproductive success. This is a distinctive feature that sets tapeworms apart from other flatworms or related organisms.

While flatworms do include a variety of forms, not all flatworms possess proglottids; this structure is specifically characteristic of tapeworms. Other organisms listed, such as sponges and mollusks, do not have any form of proglottids as they belong to entirely different phyla and have different forms of body structure and reproduction.