Figure 12.12: An echinoderm, the keyhole sand dollar. ( 18 )Theseandotheradaptationswillbeexploredinmoredetailasweexaminethismostfascinat-
ing group of invertebrates. Next time you take a walk on the beach, you’ll have appreciation
for these organisms and how they are adapted for their environment.
What are Echinoderms?
Echinoderms belong to the phylumEchinodermata, which contains marine animals living
at all ocean depths. It consists of about 7,000 living species, the largest phylum without
freshwater or terrestrial members. Also, few other groups are so abundant in the deep ocean
as well as the shallower seas.
As mentioned earlier, echinoderms are radially symmetric. In spite of their appearance,
they do not have an external skeleton. Instead, a thin outermost skin covers an internal
endoskeleton made of tiny calcified plates and spines, contained within tissues of the or-
ganism, and which forms a rigid support. Some groups, such as the sea urchins (Figure
12.13), have calcareous spines, referred to earlier, which protect the organism from predation
and colonization by encrusting (covering or coating) organisms. The sea cucumbers also use
these spines for locomotion.
Echinoderms have a uniquewater vascular system, a network of fluid-filled canals, which
function in gas exchange, feeding, and also in locomotion. This system allows them to
function without gill slits found in other organisms. Echinoderms possess a very simple