The phylum Echinodermata includes radially symmetrical marine animals such as sand dollars, sea cucumbers, sea starts, and sea urchins. Modes of obtaining food vary between different echinoderms. Brittle stars filter feed, sea urchins feed on algae, sea cucumbers capture and filter through their food, and starfish are carnivores. To digest their food, echinoderms possess simply digestive systems that are specialized towards their diets. Starfish contain a mouth, esophagus, two part stomach, intestine, and rectum. Brittle stars have a digestive tract with no intestine or anus, and waste is expelled from the mouth. Sea urchins feed on algae and use specialized mouthparts to graze the algae from rocks; its digestive tract consists of a pharynx, esophagus, small intestine, large intestine, rectum, and anus.
Pisaster brevispinus (Pink Sea Star) The Pink Sea Star feeds on bivalves, snails, micro algae, and decaying organic material. To obtain its food, specialized tube feet dig into the ground to extract bivalves, and then the Pink sea star proceeds to pry open the bivalve's shell and then inserts its stomach into the shell. The soft tissues inside of the shell are then digested. When the Pink sea star obtains food from its mouth, the food then goes to the esophagus and into the stomach. Its stomach is divided into two sections. The cardiac portion of the stomach is larger and able to be pulled back after being everted to catch prey. The smaller pyloric section secretes digestive enzymes and absorbs nutrients from the food. From the pyloric stomach, the digested material travels to the small intestine and rectum where wastes are formed. From the rectum, the waste is expelled from the Pink sea star's anus.
Echinus melo (Melon Sea Urchin) The Melon Sea urchin feeds on algae and small invertebrates. To obtain its food, the mouth of the Melon Sea Urchin contains five pointed teeth and specialized mouth parts designed to graze on algae from rocks. Once food enters the Melon Sea Urchin's mouth, cilia pull strings of mucus containing food particles into its pharynx. The muscular pharynx then opens to the esophagus from which the food travels to the small intestine which loops around to join the large intestine. Digestion occurs in the Melon Sea Urchin's intestine, when digestive enzymes are released to break down the food. A siphon runs alongside the intestine to collect nutrients and water from the ingested food. From the large intestine, wastes move to the rectum and are then expelled from the anus.
Parastichopus californicus (Red Sea Cucumber) The Red Sea Cucumber feeds on plankton, decaying material, and algae. To obtain its food, it positions itself in a current to catch prey or food particles in its tentacles. It also uses specialized mouthparts to eat algae from rocks. Once the food enters the mouth, it moves to the pharynx which contains ten rings of calcium plates that push food throughout the pharynx. From the pharynx, the food travels to the esophagus and then to the stomach where digestive enzymes are released to break down the food. From the stomach, the food enters the long and coiled intestine which loops throughout the sea cucumber's body three times; in the intestine, more enzymes are released to further break down the food, and nutrients are released. The waste is then expelled from its anus.