Which organisms belong to the annelid group?

The annelid group includes organisms that are characterized by their segmented bodies. This group primarily consists of worms and leeches, which exhibit distinct body segments that make them unique among invertebrate animals. These segments allow for greater flexibility and mobility, and many annelids also have a closed circulatory system and a coelom (a fluid-filled body cavity).

Worms, such as earthworms and marine polychaetes, play essential roles in soil aeration and nutrient cycling, while leeches are often found in aquatic environments and can have various ecological roles. This segmentation and the structural characteristics of these organisms clearly identify them as members of the annelid phylum.

In contrast, the other choices include organisms from different phyla. Mollusks and crabs belong to the Mollusca and Arthropoda phyla, respectively, while spiders and insects are also from the Arthropoda phylum, and starfish and sea urchins belong to the Echinodermata phylum. Each of these groups has distinct biological and anatomical traits that differentiate them from annelids.