ECHIXODERMATA. 



61 



Fie:. 40. 



A few dried unstalked Crinoids are shown in Table-case 1a, and 

 some preserved in spirit in Wall-case IV. ; these show the leading 

 modifications of structure in the two great genera Antedon and 

 Actinometra. 



Asteroidea. — This Order comprises Echinoderms with a de- 

 pressed body of pentagonal or star-like shape, to the ventral surface 

 of which the ambulacral feet are con- 

 fined. The rays are more or less elongate 

 movable arms, with skeletal structures, 

 which consist of transversely arranged, 

 paired, calcareous plates, articulated with 

 each other like vertebras, the series ex- 

 tending from the mouth to the end of 

 the arms. The groove in which the 

 ambulacral feet are arranged is un- 

 covered. 



Typical specimens of this Order are 

 exhibited in Case 1, in which the great 

 variety of form in the genus Asterias 

 and beautiful examples of Acanthastcr 

 are shown. Case 2 b & c contains spe- 

 cimens illustrating the curious habit of 

 self-mutilation possessed by so many 

 Echinoderms ; among Starfishes, and 

 notably in the genus Linckia, the single 

 arms separated from the disk are able to 

 develop a fresh disk and arms, and so to multiply the species. 

 Cases 2d-f and 3 a contain fine series of Oreaster. 



Ophiuroidea, or "Brittle-stars." — These Echinoderms appear 

 to resemble the ordinary Starfish; but they differ in having the 

 organs of digestion, respiration, and reproduction confined to the 

 disk, the arms having merely the function of locomotive organs. 

 The arms therefore are more slender and cylindrical in form, and 

 are sharply distinct from the disk ; the separate joints consist of 

 two central ossicles, which leave only a narrow canal between them, 

 and these are covered above, below, and at the sides by specially 

 developed investing plates ; the lateral plates bear spines, which 



f2 



Comet form of Linckia. 



