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18.4: Echinoderms and Invertebrate Chordates

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • Summarize traits of echinoderm invertebrates.
  • Outline the characteristics and classification of chordates.
  • Describe the two subphyla of invertebrate chordates.


consists of all animals with a notochord, dorsal hollow nerve cord, post-anal tail, and pharyngeal slits during at least some stage of their life
invertebrates such as sea stars and sand dollars that are characterized by a spiny endoskeleton, radial symmetry as adults, and a water vascular system
members of the subphylum Cephalochordata
members of the subphylum Urochordata are tunicates (also called sea squirts)


The invertebrate phyla described in the first three lessons of this chapter are all nonchordates. They don’t have a notochord, and they are not closely related to chordates. In this lesson, you will read about invertebrates that are closely related to chordates—including you.


Echinoderms are marine organisms that make up the phylum Echinodermata. They can be found in the ocean from the equator to the poles. There are roughly 6000 living species of echinoderms. They are among the most distinctive organisms within the animal kingdom. Members of the phylum include sea stars (starfish), sand dollars, and feather stars, all shown in Figure below.

Examples of Echinoderms. You may have seen sea stars and sand dollars at the beach because they live in shallow water near the shore. Other echinoderms, such as feather stars, are less commonly seen because they live in the deep ocean.

Structure and Function of Echinoderms

Echinoderms are named for their “spiny skin.” However, the spines aren’t on their skin. They are part of the endoskeleton. The endoskeleton consists of calcium carbonate plates and spines, covered by a thin layer of skin. Adult echinoderms have radial symmetry. This is easy to see in the sea star and sand dollar in Figure above. However, echinoderms evolved from an ancestor with bilateral symmetry. Evidence for this is the bilateral symmetry of their larvae.

A unique feature of echinoderms is their water vascular system. This is a network of canals that extend along each body part. In most echinoderms, the canals have external projections called tube feet (see Figure below). The feet have suckers on the ends. Muscle contractions force water into the feet, causing them to extend outward. As the feet extend, they attach their suckers to new locations, farther away from their previous points of attachment. This results in a slow but powerful form of movement. The suckers are very strong. They can even be used to pry open the shells of prey.

Tube Feet of a Sea Star. The tube feet of a sea star (in white) are part of its water vascular system. There is a sucker on the end of each foot that allows the animal to “walk” slowly over a surface. The suckers are strong enough to pry open shells.

Echinoderms lack respiratory and excretory systems. Instead, the thin walls of their tube feet allow oxygen to diffuse in and wastes to diffuse out. Echinoderms also lack a centralized nervous system. They have an open circulatory system and lack a heart. On the other hand, echinoderms have a well-developed coelom and a complete digestive system. Echinoderms use pheromones to communicate with each other. They detect the chemicals with sensory cells on their body surface. Some echinoderms also have simple eyes (ocelli) that can sense light. Like annelids, echinoderms have the ability to regenerate a missing body part.

Echinoderm Reproduction

Some echinoderms can reproduce asexually by fission, but most echinoderms reproduce sexually. They generally have separate sexes and external fertilization. Eggs hatch into free-swimming larvae. The larvae undergo metamorphosis to change into the adult form. During metamorphosis, their bilateral symmetry changes to radial symmetry.

Echinoderm Classification

Living echinoderms are placed in five classes. These five classes show many similarities. Organisms in each class are described in Table below.

Class (includes) Description Example


  • feathers stars
  • sea lilies
fewer than 100 species; many have more than five arms; earliest and most primitive echinoderms; live on the ocean floor, mainly in deep water; filter feeders

feather star


  • sea stars
almost 2000 species; most have five arms; many are brightly colored; live on the ocean floor, mainly in shallow water; predators or scavengers

sea star


  • brittle stars
about 2000 species; central disk distinct from arms; move by flapping their arms, which lack suckers; live on the ocean floor in shallow or deep water; predators, scavengers, deposit feeders, or filter feeders

brittle star


  • sea urchins
  • sand dollars
  • sea biscuits
  • heart urchins
about 100 species; do not have arms but do have tube feet; have a specialized mouth part with teeth to scrape food from rocks; live on the ocean floor in shallow or deep water; predators, herbivores, or filter feeders

sea urchin


  • sea cucumbers
about 1000 species; long body without arms; unlike other echinoderms, have a respiratory system; live on the ocean floor in shallow or deep water; deposit feeders, or filter feeders

sea cucumber

Introduction to Chordates

The phylum Chordata consists of both invertebrates and vertebrates chordates. It is a large and diverse phylum. It includes some 60,000 species. Chordates range in length from about a centimeter to over 30 meters (100 feet). They live in marine, freshwater, terrestrial, and aerial habitats. They can be found from the equator to the poles. Several examples of chordates are pictured in Figure below.

Diversity of Chordates. These six species illustrate the diversity of the phylum Chordata.

Characteristics of Chordates

Chordates have three embryonic cell layers. They also have a segmented body with a coelom and bilateral symmetry. Chordates have a complete digestive system and a closed circulatory system. Their nervous system is centralized. There are four additional traits that are unique to chordates. These four traits, shown in Figure below, define the chordate phylum.

  • Post-anal tail: The tail is opposite the head and extends past the anus.
  • Dorsal hollow nerve cord: The nerve cord runs along the top, or dorsal, side of the animal. (In nonchordate animals, the nerve cord is solid and runs along the bottom).
  • Notochord: The notochord lies between the dorsal nerve cord and the digestive tract. It provides stiffness to counterbalance the pull of muscles.
  • Pharyngeal slits: Pharyngeal slits are located in the pharynx. This is the tube that joins the mouth to the digestive and respiratory tracts.

Body Plan of a Typical Chordate. The body plan of a chordate includes a post-anal tail, notochord, dorsal hollow nerve cord, and pharyngeal slits.

In some chordates, all four traits persist throughout life and serve important functions. However, in many chordates, including humans, all four traits are present only during the embryonic stage. After that, some of the traits disappear or develop into other organs. For example, in humans, pharyngeal slits are present in embryos and later develop into the middle ear.

Classification of Chordates

Living species of chordates are classified into three major subphyla: Vertebrata, Urochordata, and Cephalochordata. Vertebrates are all chordates that have a backbone. The other two subphyla are invertebrate chordates that lack a backbone.

Invertebrate Chordates

Members of the subphylum Urochordata are tunicates (also called sea squirts). Members of the subphylum Cephalochordata are lancelets. Both tunicates and lancelets are small and primitive. They are probably similar to the earliest chordates that evolved more than 500 million years ago.


There are about 3,000 living species of tunicates (see Figure below). They inhabit shallow marine waters. Larval tunicates are free-swimming. They have all four defining chordate traits. Adult tunicates are sessile. They no longer have a notochord or post-anal tail.

Tunicates (Urochordata). Tunicates are one of two subphyla of invertebrate chordates.

Adult tunicates are barrel-shaped. They have two openings that siphon water into and out of the body. The flow of water provides food for filter feeding. Tunicates reproduce sexually. Each individual produces both male and female gametes. However, they avoid self-fertilization. Tunicates can also reproduce asexually by budding.


There are only about 25 living species of lancelets. They inhabit the ocean floor where the water is shallow. Lancelet larvae are free-swimming. The adults can swim but spend most of their time buried in the sand. Like tunicates, lancelets are filter feeders. They take in water through their mouth and expel it through an opening called the atriopore (see Figure below). Lancelets reproduce sexually and have separates sexes.

Lancelet (Cephalochordata). Unlike tunicates, lancelets retain all four defining chordate traits in the adult stage. Can you find them?

Lesson Summary

  • Echinoderms are marine invertebrates. They include sea stars, sand dollars, and feather stars. They have a spiny endoskeleton. They have radial symmetry as adults but bilateral symmetry as larvae. Echinoderms have a unique water vascular system with tube feet. This allows slow but powerful movement.
  • Chordates include vertebrates and invertebrates that have a notochord. Chordates also have a post-anal tail, dorsal hollow nerve cord, and pharyngeal slits. Vertebrate chordates have a backbone. Invertebrate chordates do not. Invertebrate chordates include tunicates and lancelets. Both are primitive marine organisms.

Lesson Review Questions


1. Describe the echinoderm endoskeleton.

2. Give an example of an organism in each class of living echinoderms.

3. Identify the four defining traits of chordates.

4. Name and describe the two subphyla of invertebrate chordates.

Apply Concepts

5. Create a labeled drawing that explains how the tube feet of echinoderms allow them to “walk.”

Think Critically

6. Adult sea stars and other echinoderms have obvious radial symmetry. What evidence supports the claim that echinoderms evolved from an ancestor with bilateral symmetry?

7. Adult humans lack the defining traits of chordates. Why are humans still classified in the chordate phylum?

Points to Consider

This chapter and the chapter before it describe the amazing diversity of invertebrates. The remaining chapters are devoted to vertebrates.

  • How do vertebrates differ from invertebrates? What is the main distinguishing feature of vertebrates?
  • Many traits that evolved in invertebrates characterize all vertebrate animals as well. Which invertebrate traits do you think are also found in vertebrates such as humans?

Opening image copyright Mikhail Melnikov, 2010. Used under license from Shutterstock.com.

For Table above, from top to bottom:

For Table above, from top to bottom:

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