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Learn about the digestive system of the phylum Echinodermata – invertebrates that include starfish, sea urchins, and sand dollars.

They have simple digestive systems when compared to other animals. Learn about what they eat and how they digest it.

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Diet of Echinoderms

Most of the well-known ocean invertebrates (starfish, sea urchins, sea cucumbers, brittle stars, and sea lilies) belong to the phylum Echinodermata. Unlike humans, who have well-defined stomachs, and therefore an obvious place for their digestive system to be located, echinoderms have unusual body shapes that can make locating their digestive system parts a little trickier.Before we consider the digestive system of echinoderms, it might be helpful to consider what foods the system will be digesting.

There are five main classes of echinoderms: starfish, brittle stars, sea lilies, sea cucumbers, and echinoids, which include sea urchins and sand dollars. Within those classes, we find a range of diets, ranging from carnivorous predators to vegetarian filter feeders.Most sea lilies and some brittle stars are filter feeders, extracting microscopic food particles from the water column.Sea urchins and sand dollars are mainly algae grazers, traveling around on the ocean bottom in search of various species of hard and soft algae, and occasionally eating decaying animal matter. However, for those who keep saltwater aquariums, urchins can sometimes mistake decorative coralline algae and even some corals for dinner, so in captivity their grazing can be somewhat problematic.Sea cucumbers are considered to be deposit feeders, meaning they stuff their mouths full of a deposit of sediment, which is then digested for useful nutrients and the remaining sediment excreted.

Basically, they shove a bunch of sand and sediment into their mouths and hope for the best.The majority of starfish, and some larger brittle stars, are carnivorous predators, preying on small fish and invertebrates.

The Echinoderm Digestive System

Compared to many other animals, and even other invertebrates, echinoderms have relatively simple digestive systems. Their digestive system consists of four main parts: the mouth, stomach, intestines, and anus.

Given their pentaradial symmetry and unusual body shapes, their mouths are almost always on the underside of their bodies, and the hole located at the top of their body is actually the anus. So when you are viewing an echinoderm from the side, where you are expecting its mouth and brain to be located is actually where its anus is located. Ponder that for a moment if you will…Food enters the digestive system through the mouth, just like it does with humans, and then, depending on the species, either travels directly through the pharynx into the intestines, or from the pharynx into the esophagus.In the species that have an esophagus (again, like humans), the digesting food travels from the esophagus to the first of two stomachs, known as the cardiac stomach.

Food begins to break down in the cardiac stomach with the assistance of the digestive glands, which produce digestive enzymes that aid in food breakdown and nutrient absorption. Humans have digestive enzymes too. Food is then passed into the second stomach, the pyloric stomach, where it continues to digest before being passed through the intestinal tract.Given their generally small body cavities, the intestines of echinoderms tend to be wound in a circular fashion around the main body cavity of the animal. Herbivores and filter feeders tend to have longer intestinal tracts than carnivores do. Any remaining undigested material that makes it to the end of the intestinal tract is expelled through the anus.

So you actually have a lot in common with echinoderms when it comes to your digestive system.

Variations in the Digestive System

There are a few exceptions to the general digestive pathway described in the last section, primarily in the starfish and brittle star classes. Brittle stars, for example, have what is referred to as a blind gut, or a digestive system with no intestinal tract or anus. All food enters, and all waste exits, through their mouths. Imagine how different your life might be if your mouth and your anus, and their functions, were interchangeable.

While starfish have a digestive pathway similar to what was described above, they are also able to push their cardiac stomachs outside of their bodies to allow the stomach to digest food items too large to fit inside the animal’s mouth. The food is digested externally in the cardiac stomach before being taken internally into the pyloric stomach. Just try to picture what it would be like at an all-you-can-eat buffet if humans could remove their stomachs to be able to consume larger quantities of food, and you’ll have some idea of what a starfish trying to eat an extra large meal looks like.

Lesson Summary

Members of the Echinodermata – the phylum of ocean invertebrates – can be carnivores, omnivores, or herbivores. They can be active predators or passive filter feeders. Their digestive systems are relatively similar, and include four main parts: a mouth, stomach, intestines, and anus.

Their pentaradial symmetry results in the mouth being located on the bottom surface of the animal and the anus on the top in almost all cases.Some of these creatures are filter feeders, meaning they extract microscopic food particles from the water column. Others are deposit feeders, meaning they stuff their mouths full of a deposit of sediment and pretty much hope for the best.In most species, food travels from the mouth through the esophagus to the cardiac stomach (the first stomach), to the pyloric stomach (the second stomach), to the intestinal tract, to the anus.

The cardiac stomach works in conjunction with digestive glands that secrete digestive enzymes that help to break down food into nutrients. Members of the starfish and brittle star classes are able to take their cardiac stomachs outside of their bodies and directly insert food into them, thereby allowing them to consume food items that would otherwise be too large to fit in their mouths. Other brittle star species have what is termed a blind gut, where they have no anus, and their mouths act to both take in food and expel wastes.

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