This exercise illustrates the two major features that all members of the Phylum Echinodermata have in common - the water vascular system and pentaradial symmetry.

The take home message is that all members of a phylum are built along the same basic body plan, with each class showing modifications of this basic plan.

Representatives of the five living classes are examined for these common features and compared with one another to note how these basic features are modified in each class.

The water vascular system may serve three functions in the phylum:

(a) for respiration and excretion

(b) for locomotion

(c) for food capture.

Class Crinoidea - sea lilies.

- has the most ancestral body organization of all living echinoderms.

- possess pentaradial symmetry with the 10 arms, which it uses to swim.

- the mouth is located on the upper (dorsal) surface, at the base of the arms.

- the water vascular system functions in excretion and respiration.

- there is single row of suckerless tube feet down each arm.

- it is a filter-feeder, catching plankton on the sticky tube feet and passing them into a groove running  down the length of the arm, where cilia move them in a mucus string down to the mouth.

Class Asteroidea - By turning the crinoid upside down and joining the arms together in pairs, you                   approximate a seastar.

- the arms are usually in multiples of 5.

- the mouth is ventral.

- there are tube feet (two rows per arm).

Seastars are major intertidal predators and use their tube feet for locomotion and to pull bivalves apart. They have an eversible stomach for external digestion of the prey.

Class Ophiuroidea - brittle stars have a basic similarity of body design to the asteroids.

- pentaradial symmetry is visible around the mouth and in the arms.

- brittle stars have much greater flexibility in their arms than seastars. The arms are used in  locomotion by pushing and pulling. There is less strength in the arms for pulling apart bivalves so  these animals tend to feed on soft prey.

- each arm has 2 rows of tube feet like the asteroids.

Class Echinoidea - A seastar can be transformed into a sea urchin by turning up the arms to meet                   dorsally, so that the tube feet are arranged in rows on the sides of a round body                   and the mouth is still located ventrally.

- the dried test (skeleton) of sea urchins demonstrate evidence of pentaradial symmetry in the rows of  spines and tube feet.

- in the middle of the ventral surface of a living urchin is the mouth and a 5-part Aristotle's lantern  jaw used for grazing algae (kelp).

- the urchin uses its spines, which have attached muscles, and the long tube feet for movement. Tube  feet can capture bits of food and move them to the ventral mouth.

- the related sand dollar shows similarities with sea urchins. It has tiny spines and tube feet are  restricted to 5 dorsal, petaloid areas. Tube feet are not used for locomotion but function in feeding.

Class Holothuroidea - the sea cucumbers.

- sea cucumbers have lost obvious pentaradial symmetry.

- they lack a hard endoskeleton unlike other echinoderms.

- tube feet have been modified into tentacles around mouth which aid in feeding.

- 5 rows of tubercles run from mouth to anus indicate pentaradial symmetry but sea cucumbers have an  anterior and posterior orientation when they move so they are bilaterally symmetrical animals.

- possess an internal water vascular system built on the pentaradial plan.