Classification of Echinoderms

Taxonomists divide the 7,000 species of echinoderms into six classes, five of which we

will discuss

Class CrinoideaCalled crinoids, include the sea lilies and feather

stars.The name crinoid means “lily-like.”About 600 living species.Sea lilies most closely resemble the fossils of

ancestral echinoderms from the Cambrian period. They are sessile as adults, remaining attached to rocks or the sea bottom.

Sessile Free-moving

Class CrinoideaFeather stars, in contrast, can swim or crawl

as adults, although they may stay in one place for long periods.

In both types of crinoids, five arms extend from the body and branch to form many more arms – up to 200 in some feather star species. Fossil specimen

of stalked sea lily

Class CrinoideaSticky tube feet located at the end of each arm filter

small organisms from the water. The tube feet also serve as a respiration surface across which crinoids exchange oxygen and carbon dioxide with the water.

Cilia on the arms transport trapped food to the crinoid’s mouth at the base of the arms. The mouth faces up in crinoids, while in most other echinoderms the mouth faces toward the sea bottom.

Class OphiuroideaConsists of basket stars and brittle stars.Distinguished by their long narrow arms,

which allow them to move more quickly than other echinoderms.

About 2,000 species – the largest echinoderm class.

Basket stars Brittle star

Class OphiuroideaBrittle stars, so named because the coiled

branches of their flexible arms break off easily, can regenerate missing parts

Most basket stars and brittle stars are active at night and are usually found beneath stones or seaweed or buried under the sand.

Class EchinoideaConsists of about 900 species of sea urchins

and sand dollars.Echinoidea means “spinelike”Sea urchins are well adapted to live on hard

sea bottoms. They use their tube feet for locomotion and feed by scraping algae from hard surfaces with the five teeth that surround their mouth.

Underside of urchin showing teeth

Class EchinoideaIn some sea urchins, the spines are flexible,

while in others, they are hollow and contain a venom that is dangerous to predators as well as swimmers.

Pencil Sea Urchin (flexible spines)

Purple Urchin

Class EchinoideaSand dollars live along seacoasts. They are usually

found in coastal areas and have the flat, round shape of a silver dollar. Their shape is an adaptation for shallow burrowing. The short spines on a sand dollar are used in locomotion and burrowing, and they help clean the surface of the body. They use their tube feet to capture food that settles on or passes over their body.

Class HolothuroideaSea cucumbers belong to the class HolothuroideaArmless. Live on the sea bottom, where they

crawl or burrow into soft sedimentAbout 1,500 speciesThe ossicles that make up their endoskeleton are

very small and are not connected to each other, so their bodies are soft.

Class HolothuroideaModified tube feet form a fringe of tentacles

around the mouth. When these tentacles are extended, they resemble the polyp form of some cnidarians. That explains the name of this class, which means “water polyp.”

A sea cucumber uses it tentacles to sweep up sediment and water. It then stuffs its tentacles into its mouth and scrapes the food off them.

Unique defensive behavior: Can forcefully eject part of their internal organs when threatened.

Class AsteroideaThe sea stars belong to the class Asteroidea,

which means “starlike”About 1,500 speciesThey live in coastal waters all over the worldThey exist in a variety of colors and shapes

and can have dozens of armsThey are economically important because

they prey on oysters.