GENERAL HISTOLOGY

Detailed explanation-1: -Sea urchins have globular bodies covered by calcified spines. The spines are either rounded at the tip or hollow for envenomation. The also can have pedicellariae that can grasp and envenomate, typically with more venom than in the spines.

Detailed explanation-2: -Pedicellariae are small jaw-like pincher appendages found on many species of sea urchins and sea stars (Fig. 3.88). They are typically attached to the echinoderm body at the base of the spines.

Detailed explanation-3: -In life the pedicellariae are in constant motion. The valves that form the head can open and close and the primary function of the pedicellariae is to keep the surface of the test free of debris and small parasites.

Detailed explanation-4: -During such tides large numbers of urchins can be seen in the lagoon channel just below the low tide line where they dominate large areas. Sea urchins, relatives of the starfish, are plant eaters and account for the absence of algae from the channel or adjacent areas.