Echinoderm Digestive System
Echinoderms belong to a phylum of marine animals which include starfish, sea urchins, sand dollars, and sea cucumbers among 7000 different species. Echinoderms have a wide variety of feeding mechanisms, and their digestive systems vary according to the food they consume and the individual needs of each species. Echinoderms are known for their characteristic body symmetry and their unique abilities of regeneration, with certain starfish being capable of completely regrowing from a single limb. Echinoderms make use of tube feet to move them towards their food sources and often use these tube feet to attache to their prey or direct their mouth area toward the food source. Though the digestive systems vary, they are all relatively simple when put into comparison with an organism such as a human.
Protoreaster linckii (Red-Knobbed Starfish)
The Red-Knobbed starfish is an omnivore, with a diet of algae early in life when it is still small. In adulthood, the Red-Knobbed starfish has a diet ranging from sponges and tube-worms to other starfish and slow-moving or dead/dying fish. The starfish makes use of its tube feet to move and to capture or attach itself to prey. Smaller food is moved by the tube feet along the outer arms into the mouth located in the center on the underside of the starfish. Food is pushed by the esophagus into the stomach and digested by enzymes produced in the pyloric cecum. In the Red-Knobbed Starfish, the mouth is in actuality a sort of primary stomach. In cases of larger prey, the starfish will evert the this primary stomach, known as the cardiac stomach, and completely engulf its prey. The cardiac stomach will release the digestive enzymes and the prey will begin to be digested externally.Food will then make its way down the esophagus into the internal pyloric stomach for further digestion. In the intestine, the broken down nutrients are separated from wastes and absorbed while wastes are transformed into fecal matter. Waste proceeds to the rectal cecum where it is stored before being expelled through the anus. Oftentimes, however, undigested waste is regurgitated through the mouth rather than being expelled through the anus. The Red-Knobbed Starfish, as well as starfish in general, are able to consume a vide variety of organisms, often much larger than their mouths, because of their unique ability to digest food outside of their body. |
Echinus Melo (Melon Sea Urchin)
The Melon Sea Urchin is generally a herbivore, with a diet consisting mostly of algae, however, it is known to consume small invertebrates. The melon Sea Urchin makes use of tube feet on its outer surfaces to move itself towards sources of food. The urchin's mouth is located on its underside and its digestive tract ends with the anus located at the top of the sea urchin, relative to the mouth. Similarly to a starfish, the urchin uses the tube feet on its underside to direct food towards the mouth. Within the Melon Sea Urchin's mouth is an apparatus made up of five large calcium carbonate teeth in a circular formation surrounding a fleshy tongue-like structure. The Urchin uses this mechanism, which is known as Aristotle's lantern, to chew its food. The lantern is connected to both the mouth and the pharynx. Food is taken in through the mouth cavity past the pharynx and into the esophagus. The esophagus goes down from the lantern to connect to the small intestine. The small intestine travels once completely around the inside of the Urchin to connect to the large intestine, which forms another complete circle within the Urchin's body. Though named the large and small intestine, they are not separated in the same manner as the intestines within humans and form more or less one complete tube. Digestion occurs within the intestine with the use of enzymes produced by a structure known as the caecum, which is located in the small intestine. The large intestine connects to a rectum, which travels upwards and connects to the anus at the top of the sea urchin's body. Additionally, connected to the intestine is a tube known as the siphon, which takes water out of food in the digestive tract and absorbes it into the the water vascular system of the urchin. The name for Aristotle's lantern comes from a translation of Aristotle's description of the anatomy of sea urchins, where he supposedly compared the jaw apparatus to the lanterns being used in his time. |
Holothuria cinerascens (Sea Cucumber)
The sea cucumber known as Holothuria cinerascens is a species of Sea Cucumber native to the Pacific ocean. Like most sea cucumbers, it is a scavenger and its diet consists mainly of plankton and decaying organic matter. Sea cucumbers will either burrow in to sediment or postion themselves in water currents to collect food. Holothuria cinerascens uses the feeding tentacles to collect food and direct it into the mouth, which these tentacles surround. The feeding tentacles in Holothuria cinerascens are connected to a ring structure made up of ten calcium carbonate plates, the only significant bone structure. This bone structure allows the feeding tentacles to have an anchor point for muscles and creates the ability to retract the feeding tentacles into the mouth. The ring structure then connects to the pharynx. Some sea cucumbers have an esophagus and stomach, but in the case of Holothuria cinarascens, the pharynx connects directly to the intestine. Digestion mainly takes place within the intestine, which usually coils inside the body about three times. The intestine connects directly to the anus through which waste particles are expelled. Due to the simple nature of Holothuria cinerascens' diet, the digestive tract itself is also rather simple. |