PHYLUM ECHINODERMATA -- FREE-LIVING FORMS
Several classes of echinoderms do not have a stem, and most of them are able to move about on the sea-floor, often burrowing into the sediment. Four of the five surviving classes of the phylum live this way.
The most familiar echinoderms are the seastars (or 'starfish' -- Class Asteroidea), which nearly everybody has seen. Most have few mineralized skeletal parts, and even when they do, the parts are not well attached to each other. Whole fossils are pretty rare, though they do exist, but the loose pieces are more commonly found. The first photo is of a Recent species of Astropecten from an unknown locality. The large plates at the bottom are loose pieces of a related species from late Eocene rocks near Qasr el Sagha, Egypt. The scale bar is 1cm. (Photo by B. Carter.)
The next photograph shows a Recent brittle star (Class Ophiuroidea) from Lemon Bay, FL. Notice that the ophiuroids are similar in form to the seastars, but the arms are thinner and are more obviously differentiated from the central part. The scale bar is 1cm. (Photo by B. Carter.)
The most commonly fossilized of the free-living echinoderms are the echinoids (Class Echinoidea). There are two major types. The first photo shows a type called a "regular" echinoid (or 'sea urchin'), as seen from the bottom (Phyllacanthus imperialis from the Indo-Pacific region). Notice the five rows of pores that radiate from the large opening, and notice that you can see them as well radiating from the smaller hole on the opposite surface. There are also five rows of paired large knobs ('tubercles") between the pore zones. the large opening on the lower surface was the location of the mouth. The smaller one visible through it (on the upper surface) was the location of the anus. The fact that these two orifices are at opposite poles, and the fact that the pore zones radiate around the shell from one to the other is what makes these "regular". The scale bar is 1cm. (Photo by B. Carter.)
The next photograph shows a fossil relative (Prionocidaris mortoni) of the above species in side view. Notice the pore zones and the zones of large tubercles. Each tubercle had a spine, like those in the right-hand photo, attached to it.
The other main type of echinoid is called "irregular"; sand dollars are a familiar example. Irregular echinoids have the mouth somewhere on the lower surface, not usually at the center, but the anal opening is usually on the back side rather than top and center. The pore zones are usually not as obviously continuous around the shell, appearing to end somewhere on the upper surface, and the radial symmetry is overshadowed by a secondary bilateral symmetry. The next picture shows a common example of a fossil irregular echinoid from lower Oligocene rocks in the southeastern USA. This specimen of Rhyncholampas gouldii is from Brooksville, FL. The scale bar is 1cm. (Photo by B. Carter.)
