Most members of this phylum are marine bottom dwelling and relatively stationary. The scavenging habits of these invertebrates are important in keeping oceans and seas free of animal debris. Even though members of the phylum have a similar body plan, the differences between the many classes can easily be seen. Elements of the body usually are arranged in a pentameral or five-sided pattern. A skeleton of calcite encloses the bulk of the internal soft parts of echinoderms. On the surface spines and fleshy skin cover the skeletal framework. A structure termed the water vascular system is composed of sacs, tubes, and canals. This system is very important in circulating body fluids and in movement.
Sea urchin spines
Echinoid body plate
Living sea urchin
Echinoderm skeletons are composed of plates of calcite, a mineral that is easily extracted from the ocean waters and is widely used by marine organisms. Its an armor of sorts from which this phylum gets its name. In some echinoderms, these plates are united or fused together to form a rigid shell (as in sea urchins). You'll probably notice that your collection of fossil echinoderms is more made up of bits and pieces than whole, complete specimens. It's not because of anything you did in the field, an "oops factor" with your rock pick, but rather because in may echinoderms, the plates are loosely connected muscle tissue which disintegrates after the animal dies (as in crinoids). Just keep looking. You've got to kiss a lot of toads before you find your prince. The fossil record of the phylum extends from the Early Cambrian to the present.
This beautiful fossil sea urchin lost its spines
Steroids (not the muscle-building drugs) have a star-shaped body with five or a multiple number of arms projecting outward like the spokes of a wheel. On the bottom surface of a starfish the arms extend from the central mouth. In the arms lie the food grooves with many tube feet which enable them to crawl slowly over the bottom. The skeleton is composed of detached or fused plates, covered by fleshy skin and tiny spines and pincer-like structures.
Rigid biscuit-shaped armor test composed of symmetrically arranged plates characterize this group. Inside the hollow test lies the soft organs and the water circulatory system. Outside the test lies a tough covering bearing spines which aid in movement and provide some degree of protection from enemies. Echinoids lack the five rays of the starfish or the arms of crinoids, but they still retain five food grooves that lie on the upper surface of the animal. As fossils, the echinoids are first known from Middle to Upper Ordovician rocks, but are rare in Paleozoic rocks. Pennsylvanian strata in many areas of Arizona contain isolated echinoid spines and plates, but entire skeletons are rare.
Cystoids are a primitive group of echinoderms with a variable number of irregular skeletal plates composing the calyx. But in more specialized groups, such as the blastoids, the plates are few in number, but symmetrically arranged. As with most echinoderms, food grooves radiate outward from the mouth in a five-rayed pattern. Such grooves filter food out of the water and send it to the mouth and digestive tract. The class cystoides first appeared in Middle Ordovician times. The highly specialized blastoids outlived other cystoids but became extinct at the close of the Permian.
Often well preserved, blastoids can be such little gems
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