A reader recently asked us why annelid anatomy could be described as a “tube within a tube”.  Before diving in to answer this specific question, we will provide a little background information about annelids.

Annelids are often referred to as segmented worms, and there are over 10,000 species within the phylum. Annelids can be distinguished from other worms because they are segmented, belong to the Lophotrochozoa group, and have a coelom. The coelom is also known as the body cavity, and it is present in most multicellular animals. It begins at the mouth of the animal and ends at the anus.

This coelom is why the annelid anatomy can be described as a “tube within a tube”. The coelom is the inner “tube” that runs through the entire body of the annelid, and the outer “tube” is the worm body itself, which resembles a tube in shape and structure. Furthermore, there are many species that don’t have a coelom, like flatworms and cnidarians.

In sum, annelid anatomy can be described as a “tube within a tube” because of the presence of a coelom, and the shape of the worm.