SECTION IX 

 PHYLUM ECHINODERMATA 



THE phylum EcMnodermata comprises the Starfishes (Asteroidea), 

 Sea-urchins (Echinoidea), Brittle-stars (Opkiuroidea), Feather-stars 

 (Grinmdea), and Sea-cucumbers (Holothuroidea). All exhibit a 

 radial arrangement of parts, which is recognisable as well in the 

 globular Sea-urchins and elongated Sea-cucumbers, as in the star- 

 sh:tped Starfishes, Brittle-stars and Feather-stars. Another uni- 

 versal feature is the presence of a calcareous exoskeleton, sometimes 

 in the form of definitely shaped plates, which may fit together by 

 their edges so as to form a continuous shell ; sometimes merely in 

 the form of scattered particles or spicules. In very many the 

 surface is beset with tubercles or spines, from which feature the 

 name of the phylum is derived. The various systems of organs 

 attain a comparatively high degree of complexity. The Echino- 

 di-rms are rarely capable of rapid locomotion, and are sometimes 

 permanently fixed by means of a stalk : they never give rise to 

 colonies by budding. Without a single exception, all the members 

 of this phylum are inhabitants of the sea. 



1. EXAMPLE OF THE ASTEROIDEA. 

 A Starfish (Asterias rubens or Anthenea flavescens). 



General External Features of Asterias rubens. The 



body of the Starfish is enclosed in a tough, hard integument, 

 containing numerous plates, or ossicles as they are termed, of 

 calcareous material. This exoskeleton is not completely rigid in 

 the fresh condition, but presents a certain limited degree of flexi- 

 bility. The body (Fig. 276) is star-shaped, consisting of a central 

 part, the centred disc, and five symmetrically arranged processes, 

 the arms or rays, which, broad at the base, taper slightly towards 

 their outer extremities. There are two surfaces one. the dorsal . 



