Sea urchins are globular, spiny marine invertebrates belonging to the phylum Echinodermata, which also includes sea stars and sea cucumbers. They inhabit seabeds across all the world’s oceans. To answer the most immediate question, yes, a sea urchin does have a mouth. Their entire body structure and lifestyle revolve around this feeding opening and the specialized apparatus it houses.
The Location and Purpose of the Mouth
The mouth of a sea urchin is positioned on the underside of its body, known as the ventral or oral surface. This downward-facing orientation allows the sea urchin to directly graze on food particles and algae while moving across the seafloor. The opening is covered by the peristome, a soft, flexible membrane that surrounds the mouth and may contain small, embedded bony pieces. The mouth acts as the primary entry point for sustenance, and the peristomial area also features five pairs of modified tube feet and, in many species, five pairs of external gills used for respiration.
The Unique Feeding Apparatus
Within the mouth opening lies a musculo-skeletal structure known scientifically as the masticatory apparatus, commonly called Aristotle’s Lantern. This name originated from the Greek philosopher who noted its resemblance to five-sided horn lanterns. The Lantern functions as a powerful, retractable chewing and scraping organ.
The Lantern is composed of five strong, calcareous plates, often described as jaws or pyramids, held together by muscles and ligaments. Each plate houses a single, sharp tooth pointing toward the center of the mouth. The entire apparatus can be extended and retracted by muscular action, allowing precise feeding movements.
The teeth are made of calcium carbonate and are continuously growing. As the teeth wear down from constant scraping against hard surfaces, new material forms at the top, ensuring they remain sharp and effective. This self-sharpening mechanism allows the sea urchin to scrape tough algae and detritus off rocks. Some species can even bore into rock or coral to create protective depressions.
Diet and Internal Digestive Process
Sea urchins are predominantly herbivores, feeding mainly on plant matter such as algae, kelp, and organic detritus found on the seafloor. While their primary role is that of a grazer, some species may also consume slow-moving organisms. Once food is scraped by the Lantern’s teeth, it passes into the pharynx and then into the esophagus.
The food then enters a long, coiled intestine that makes an extensive circuit around the inside of the test, the sea urchin’s hard shell. This extended length is where the majority of digestion and nutrient absorption takes place. The digestive process requires the food to remain in the gut for a prolonged period to extract sufficient nutrition.
An additional tube, called the siphon, runs alongside a portion of the intestine in some species and aids in water resorption from the ingested food. After the looping path, the intestine leads to a short rectum. Indigestible waste is finally expelled through the anus, also known as the periproct, which is located on the aboral surface, or the top of the sea urchin’s body.