Sea Urchins and Sand Dollars Sea urchins and sand dollars, class Echinoidea, are the spiniest of the echinoderms. Their spines provide an excellent case study of form and function as they vary from the large, thick spines of pencil urchins to the tiny flattened spines of sand dollars. Dispersed among the spines are the pinching claws, called pedicellaria, seen in dramatic action in response to a predatory sea star. The sea urchin life cycle is followed from fertilization, through the blastula and gastrula stages, through several bilateral stages, and then through metamorphosis to the juvenile. Sea Cucumbers Sea cucumbers, class Holothuroidea, are worm-like, but their echinoderm inheritance is revealed with a morphing animation.
Sea cucumbers are suspension, or detritus, feeders.
Although seemingly vulnerable, sea cucumbers exhibit a variety of defense mechanisms, including poisons in the skin. In some species, sticky tubes that can be ejected out the anus. Feather Stars Feather stars, class Crinoidea, are most abundant on tropical coral reefs. Unlike sea stars and sea urchins, they have a mouth that is directed upward. This group is one of the oldest living classes of echinoderms, as shown by the abundance of crinoid fossils in rocks 400 million years old and older.