ECHINODERMS 



ECHINODERMA 



The Echinoderms are radially symmetrical, mostly five-radiate, 

 marine animals, with a skeleton consisting of calcareous plates 

 and with tube-feet which are in connection with an interior water- 

 vascular system. They never form colonies. 



Common to all echinoderms is the radiate structure, as 

 expressed in the arrangement of the tube-feet. The surface of 

 the body is thereby divided into (usualh^) 10 sections, alternately 

 carrying or devoid of tube-feet. The sections with the tube-feet 

 are termed Radii ( Ambulacra), the other sections Interradii 

 {Inter ambulacra). The internal organs also are wholly or partly 

 of radiate structure (watervascular, nerve, and haemal systems, 

 genital organs) ; the gut is not radiate in structure. 



The shai^ie of the body is exceedingly varied. The radii may 

 be free from each other, forming freely movable arms, the 

 body thus becoming star-shaped, as in the Sea-stars (Asteroids) 

 and Brittle-stars (Ophiuroids), or flower-shaped, as in the Sea-lilies 

 (Crinoids), or they may be intimately connected throughout 

 their whole length, the body becoming round, arched, as in 

 the Sea-urchins (Echinoids), or elongate, wormlike, or sausage- 

 shaped, as in the Sea-cucumbers (Holothurioids). 



The number of the radii is usually 5 ; exceptionally 4- or 

 6-rayed specimens of normalh^ S-raj^ed species may occur. 

 Some forms, especially among the sea-stars, are normally pluri- 

 radiate. In most sea-lilies the arms are dichotomous at the 

 base ; these thus become 10-armed, or, when the forkings are 

 repeated, manj^-armed. In some brittle-stars (Gorgonocephalus) 

 the arms are branched and tree-shaped. 



The skeleton (except in the Holothurians) is strongly developed, 

 generally forming a firm coat of regularly arranged plates, which 

 in some forms (especially the brittle-stars) have the shape of 

 vertebrae. In the Holothurians the skeleton is mostlv little 



