Echinodermata Digestive System

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  • 0:03 Diet of Echinoderms
  • 1:52 The Digestive System
  • 3:41 Variations In the…
  • 4:44 Lesson Summary
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Lesson Transcript
Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

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.

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.

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