Info

Acorn worms and pterobranchs

Chordata: Chordates

50,000

Urochordata: Sea squirts Cephalochordata: Lancelets Agnatha: Lampreys, hagfishes Chondrichthyes: Cartilaginous fishes Osteichthyes: Bony fishes Amphibia: Amphibians Reptilia: Reptiles Aves: Birds Mammalia: Mammals

* The position of this phylum is uncertain. Many researchers place them in the deuterostomes.

* The position of this phylum is uncertain. Many researchers place them in the deuterostomes.

Adaptation Bohadschia Argus

(c) Bohadschia argus

(d) Henricia leviuscula

34.4 Diversity among the Echinoderms (a) The flexible arms of this golden feather star are clearly visible. (b) Purple sea urchins are important grazers on algae in the intertidal zone of the Pacific Coast of North America. (c) This sea cucumber lives on rocky substrata in the seas around Papua New Guinea. (d) The blood sea star is typical of many sea stars; some species, however, have more than five arms. (e) The arms of the brittle star are composed of hard but flexible plates.

(c) Bohadschia argus

Ophiothrix Spiculata
(e) Ophiothrix spiculata

(d) Henricia leviuscula

34.4 Diversity among the Echinoderms (a) The flexible arms of this golden feather star are clearly visible. (b) Purple sea urchins are important grazers on algae in the intertidal zone of the Pacific Coast of North America. (c) This sea cucumber lives on rocky substrata in the seas around Papua New Guinea. (d) The blood sea star is typical of many sea stars; some species, however, have more than five arms. (e) The arms of the brittle star are composed of hard but flexible plates.

cup. The tube foot is moved by expansion and contraction of the circular and longitudinal muscles of the tube.

Many sea stars prey on polychaetes, gastropods, bivalves, and fish. They are important predators in many marine environments, such as coral reefs and rocky intertidal zones. With hundreds of tube feet acting simultaneously, a sea star can exert an enormous and continuous force. It can grasp a clam in its arms, anchor the arms with its tube feet, and, by steady contraction of the muscles in the arms, gradually exhaust the muscles the clam uses to keep its shell closed. Sea stars that feed on bivalves are able to push the stomach out through the mouth and then through the narrow space between the two halves of the bivalve's shell. The stomach secretes enzymes that digest the prey.

Brittle stars (class Ophiuroidea) are similar in structure to sea stars, but their flexible arms are composed of jointed hard plates (Figure 34.4e). Brittle stars generally have five arms, but each arm may branch a number of times. Most of the 2,000 species of brittle stars ingest particles from the upper regions of sediments and assimilate the organic material from them, but some species remove suspended food particles from the water; others capture small animals. Brittle stars eject the indigestible particles through their mouths because, unlike most other echinoderms, they have only one opening to the digestive tract.

An additional group, the sea daisies (class Concentricy-cloidea) were discovered only in 1986, and little is known about them. They have tiny disc-shaped bodies with a ring of marginal spines, and two ring canals, but no arms. Sea daisies are found on rotting wood in ocean waters. They apparently eat prokaryotes, which they digest outside their bodies and absorb either through a membrane that covers the oral surface or via a shallow, saclike stomach. Recent molecular data suggest that they are greatly modified sea stars.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment