Starfish - Internal Anatomy - Digestive System

Digestive System

The mouth of a starfish is located in the centre of the oral surface and opens through a short oesophagus into firstly a cardiac stomach, and then, a second, pyloric stomach. Each arm also contains two pyloric caeca, long hollow tubes branching outwards from the pyloric stomach. Each pyloric caecum is lined by a series of digestive glands, which secrete digestive enzymes and absorb nutrients from the food. A short intestine runs from the upper surface of the pyloric stomach to open at an anus near the centre of the upper body.

Many starfish, such as Astropecten and Luidia, swallow their prey whole, and start to digest it in the stomachs before passing it into the pyloric caeca. However, in a great many species, the cardiac stomach can be everted from the organism's body to engulf and digest food. In these species, the cardiac stomach fetches the prey, and then passes it to the pyloric stomach, which always remains internal. Waste is excreted through the anus on the aboral surface of the body.

Because of this ability to digest food outside its body, the starfish is able to hunt prey much larger than its mouth would otherwise allow. Their diet includes clams and oysters, arthropods, small fish and gastropod molluscs. Some starfish are not pure carnivores and may supplement their diets with algae or organic detritus. Some of these species are grazers but others trap food particles from the water in sticky mucus strands that can be swept towards the mouth along ciliated grooves.