SECTION IX 



PHYLUM ECHINODERMATA 



THE phylum Echinodermata comprises the Starfishes (Asteroidea), 

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

 (Crinoidea), 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- 

 shaped 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. An extensive 

 coelome is present, developed in the embryo from hollow outgrowths 

 from the archenteron. The Echinoderms are rarely capable of 

 rapid locomotion, and are sometimes permanently fixed by means 

 of a stalk ; they never give rise to colonies by budding. With 

 hardly 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. 307) is star-shaped, consisting of a central 

 part, the central 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 aboral 

 or abactinal, directed upwards in the natural position of the living 



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