Learning Outcomes

Describe the distinguishing attributes of echinodermsIdentify the different classes in phylum Echinodermata

Characteristics the Echinoderms

Echinodermata space so named owing to their spiny skin (from the Greek “echinos” meaning “spiny” and also “dermos” definition “skin”), and also this phylum is a arsenal of around 7,000 defined living species. Echinodermata are solely marine organisms. Sea stars (Figure 1), sea cucumbers, sea urchins, sand dollars, and also brittle stars room all instances of echinoderms. Come date, no freshwater or terrestrial echinoderms are known.

You are watching: Which of the following is not an echinoderm?

Figure 1. This diagram shows the anatomy of a sea star.

Morphology and Anatomy

Adult echinoderms exhibition pentaradial symmetry and have a calcareous endoskeleton made of ossicles, back the beforehand larval step of all echinoderms have bilateral symmetry. The endoskeleton is emerged by epidermal cells and also may own pigment cells, giving vivid colour to these animals, and cells laden through toxins. Gonads are existing in every arm. In echinoderms like sea stars, every arm bears two rows of tube feet top top the oral side. These tube feet assist in attachments to the substratum. These pets possess a true coelom the is modified into a unique circulatory system called a water vascular system. An exciting feature that these animals is their strength to regenerate, also when over 75 percent of their body massive is lost.

Water Vascular System

Echinoderms own a distinct ambulacral or water vascular system, consist of of a main ring canal and also radial canals that expand along every arm. Water circulates v these structures and also facilitates gaseous exchange as well as nutrition, predation, and also locomotion. The water vascular system additionally projects from feet in the skeleton in the type of tube feet. These pipe feet can broaden or contract based on the volume of water current in the mechanism of the arm. By making use of hydrostatic pressure, the pet can one of two people protrude or retract the tube feet. Water start the madreporite ~ above the aboral side of the echinoderm. From there, it passes right into the stone canal, which moves water into the ring canal. The ring canal associate the radial canals (there are 5 in a pentaradial animal), and also the radial canals relocate water right into the ampullae, which have tube feet v which the water moves. By moving water v the distinctive water vascular system, the echinoderm can move and force open mollusk shells during feeding.

Nervous System

The nervous device in these animals is a relatively simple structure with a nerve ring at the center and five radial nerves expanding outward follow me the arms. Structures analogous come a mind or acquired from blend of ganglia room not existing in these animals.

Excretory System

Podocytes, cells devoted for ultrafiltration the bodily fluids, are current near the facility of echinoderms. This podocytes are associated by an interior system of canals to an opening dubbed the madreporite.


Echinoderms are sexually dimorphic and release their eggs and sperm cells right into water; fertilization is external. In some species, the larvae divide asexually and multiply before they reach sexual maturity. Echinoderms may also reproduce asexually, and regenerate human body parts lost in trauma.

Classes of Echinoderms

This phylum is separated into five extant classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and also sand dollars), Crinoidea (sea lilies or feather stars), and Holothuroidea (sea cucumbers) (Figure 2).

Figure 2. Various members the Echinodermata encompass the (a) sea star of class Asteroidea, (b) the brittle star of course Ophiuroidea, (c) the sea urchins of course Echinoidea, (d) the sea lilies belong to course Crinoidea, and (d) sea cucumbers, representing course Holothuroidea. (credit a: modification of occupational by Adrian Pingstone; credit b: alteration of work-related by Joshua Ganderson; credit transaction c: modification of work-related by Samuel Chow; credit transaction d: modification of job-related by buy it Depper; credit e: modification of work by Ed Bierman)

The most renowned echinoderms room members of course Asteroidea, or sea stars. They come in a large variety that shapes, colors, and also sizes, with an ext than 1,800 types known therefore far. The vital characteristic the sea stars that distinguishes lock from other echinoderm classes has thick arms (ambulacra) that prolong from a main disk whereby organs penetrate right into the arms. Sea stars use their tube feet not just for gripping surfaces but also for grasping prey. Sea stars have actually two stomachs, one of which have the right to protrude v their mouths and also secrete cradle juices into or top top prey, even prior to ingestion. This procedure can essentially liquefy the prey and also make cradle easier.

Explore the sea star’s human body plan up close, clock one move across the sea floor, and also see the devour a mussel.

See more: Star Ocean Integrity And Faithlessness World Map, Star Ocean: Integrity And Faithlessness

Brittle stars belong come the class Ophiuroidea. Unequal sea stars, which have actually plump arms, brittle stars have actually long, slim arms that are sharply demarcated native the main disk. Brittle stars relocate by lashing the end their eight or wrapping them about objects and pulling themselves forward. Sea urchins and sand dollars are examples of Echinoidea. This echinoderms do not have actually arms, but are hemispherical or flattened with 5 rows of tube feet that assist them in slow-moving movement; tube feet space extruded with pores of a constant internal shell dubbed a test. Sea lilies and also feather stars are examples of Crinoidea. Both the these types are suspension feeders. Sea cucumbers of course Holothuroidea are expanded in the oral-aboral axis and have five rows of tube feet. These space the only echinoderms that demonstrate “functional” bilateral symmetry together adults, since the uniquely extended oral-aboral axis compels the animal to lie horizontally rather than was standing vertically.