Echinoderms Pentaradial Symmetry

During the evolution of one deutero stomate lineage, the echinoderms (phylum Echinodermata), two major structural features arose. One was a system of calcified internal plates covered by thin layers of skin and some muscles. The calcified plates of early echinoderms later became enlarged and thickened until they fused inside the entire body, giving rise to an internal skeleton.

The other feature was a water vascular system, a network of water-filled canals leading to extensions called tube feet. This system functions in gas exchange, locomotion, and feeding (Figure 34.3a). Seawater enters the system through a perforated madreporite. A calcified canal leads from the madrepo-rite to another canal that rings the esophagus (the tube leading from the mouth to the stomach). Other canals radiate from this ring canal, extending through the arms (in species that have arms) and connecting with the tube feet.

The development of these two structural innovations resulted in a striking evolutionary radiation. About 23 classes of echinoderms, of which only 6 survive today, have been described from fossils. The 13,000 species described from their fossil remains are probably only a small fraction of those that actually lived. Nearly all 7,000 species that survive today live only in marine environments. Some have bilaterally symmetrical, ciliated larvae (Figure 34.3b) that feed for some time as planktonic organisms before settling and transforming into adults with pentaradial symmetry (symmetry in five or multiples of five).

Living echinoderms are members of two lineages: sub-phylum Pelmatozoa and subphylum Eleutherozoa. These two groups differ in the form of their water vascular systems.

Radial symmetry, calcified plates

Deuterostomate ancestor

34.2 A Current Phylogeny of the Deuterostomes There are fewer major lineages, and many fewer species, of deuterostomes than of protostomes.

34.2 A Current Phylogeny of the Deuterostomes There are fewer major lineages, and many fewer species, of deuterostomes than of protostomes.

Radial symmetry, calcified plates

Deuterostomate ancestor

Bilateral Symmetry Sea Star

Bilateral symmetry, pharyngeal slits, notochord

Vertebral'''' column

Bilateral symmetry, pharyngeal slits, notochord

Vertebral'''' column

(a) Adult sea star (radial symmetry)

Madreporite

(a) Adult sea star (radial symmetry)

Madreporite

Pentaradial Symmetry

Tube foot

Each arm has a full complement of organs. This arm has been drawn with the digestive glands removed to show the organs lying below.

(b) Sea star larva

(bilateral symmetry)

Skin gill

Tube foot

Each arm has a full complement of organs. This arm has been drawn with the digestive glands removed to show the organs lying below.

34.3 Echinoderms Display Two Evolutionary Innovations

(a) A dorsal view of a sea star displays the canals and tube feet of the echinoderm water vascular system, as well as a calcified internal skeleton. (b) The ciliated sea star larva has bilateral symmetry.

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