Starfish, correctly known as sea stars, are marine invertebrates belonging to the phylum Echinodermata, alongside sea urchins and sea cucumbers. Their unique biology is defined by pentaradial symmetry, meaning their body parts are arranged in five sections around a central disk. This radial design dictates an internal anatomy that challenges the typical understanding of orientation, leading to the unusual location of their digestive exit.
The Aboral Surface: Locating the Starfish Anus
The location of the starfish’s anus is directly related to its radial body plan, situated on the side opposite the mouth. The sea star has two primary surfaces: the oral surface, which faces downward and contains the mouth, and the aboral surface, which faces upward. The anus is a tiny, inconspicuous opening located on this upper, or aboral, surface.
It is positioned near the center of the animal’s central disk, the main body area from which the arms radiate. This location is usually close to the madreporite, a small, sieve-like plate that takes water into the animal’s water vascular system. The anus is so small and often difficult to see because, in many species, it is not the primary mechanism for expelling waste.
The Complete Digestive Pathway
The digestive journey begins on the underside, where the mouth is located at the center of the oral surface. Food passes from the mouth into a short esophagus, which leads to the stomach, a structure divided into two main parts. The lower, or cardiac, stomach is a highly muscular sac that can be pushed out of the mouth—a process called eversion—to externally engulf and digest prey.
This external digestion allows the sea star to consume prey much larger than its mouth opening, such as clams and oysters, by dissolving the soft tissues outside its body. The partially digested material is then drawn back into the body to the smaller, upper portion, the pyloric stomach. Ducts lead from the pyloric stomach to the pyloric ceca, or digestive glands, which extend down into each of the arms and are responsible for the final absorption of nutrients. A short intestine then connects the pyloric stomach to the anus on the aboral surface.
Species Without an Anus and Other Digestive Quirks
The short, simple nature of the starfish intestine and the small size of the anus reflect its limited role in waste elimination. Because the cardiac stomach can evert, much of the large, hard, or undigested material, like shell fragments, is simply regurgitated back out through the mouth. This means the waste that reaches the anus is mainly fine, processed residue from the deep internal digestion.
This rudimentary structure is not universal, as some primitive starfish species, such as those in the genus Luidia, lack an anus entirely. In these species, all waste must be expelled through the mouth, making the mouth a dual-purpose opening for both feeding and egestion. This characteristic of a “blind gut” is also found in other related echinoderms, such as brittle stars (Ophiuroidea). The evolutionary reduction or loss of the anus highlights an adaptation where specialized external feeding removes the need for a complex exit structure.