ABSTRACT
The sea urchin test is composed of individual plates linked to each other in vertical and horizontal sutures either by connective tissue forming a flexible test or by skeletal processes and collagen fibres forming a rigid test. Musculature attached to the internal stereom of the test is restricted to: 1) ambulacral, peristomial and lantern muscles, e.g. in flexible sea urchins: 2) peristomial and lantern muscles, e.g. in rigid sea urchins with lantern and teeth; and 3) peristomial muscles, e.g. in rigid sea urchins without lantern and teeth. The internal muscles in the flexible test function as a pump. The lantern muscles function as a pump and take part in the feeding mechanism. The respiratory organs in sea urchins are specialized tube feet adapically situated on the test, and (’gills') orally situated on the test. One of the functions of the internal musculature in sea urchins is to pump oxygenated fluid from the respiratory organs to different parts of the body. In addition, the lantern muscles are part of the feeding mechanism.
