Antedon

Antedon. Though not a good representative of the Crinoidea (q.v.), A. bifida is usually selected for study, as the only easily procurable species of the class. In its larval or "Pentacrinus" stage it is fixed by a short jointed stem, as in the typical Crinoids, but it soon becomes detached from this, and is free-swimming for the rest of its existence. The adult antedon consists of a disc, giving off ten arms fringed with pinnules (small branches of the arms), and bearing below a number of short processes known as cirri. The mouth and anus both open in the upper surface of the disc; the former is central; the alimentary canal consists of a single coil. The nervous, water-vascular and blood-vascular systems (for descriptions see Crinoidea) each consist mainly of a ring round the mouth from which a branch runs up each arm; from the blood-vascular ring a vessel runs down to a "chambered organ" placed at the bottom of the cavity of the disc; round this is a second nervous centre giving off cords through the arms. The communication of the water-vascular system with the exterior is effected through a large number of pores, scattered over the body, instead of being collected into one plate as in the sea urchins. The skeleton is composed of a central plate bearing the cirri, which is surrounded by two rings of five plates (basals and radials), of which the outer bears ten arms; the arms are composed of many small joints. Antedon bifida is fairly common in many places round the English coast. Its popular name is the "Rosy Feather Star," and it is often known as A. rosacea.