IX 



PHYLUM ECHINODERMATA 



405 



4. THE CRINOIDEA. 

 A Feat her- Star. Antedon rosacca. 



General External Features. In the Feather-Star (Fig. 330), 

 as in the Starfish, there are to be recognised a central disc and a 

 series of five radiating arms. In the natural position of the animal 

 the side of the disc which corresponds to the oral or actinal 

 surface of the Starfish is directed upwards, and the aboral or 

 abactinal surface downwards. The five arms are bifurcated at 

 their bases ; they are feather-like and highly flexible, acting as 

 the locomotive organs of the animal, their alternate flexions and 

 extensions resulting in a slow movement through the water. On 



FIG. 330. Antedon. Side view of entire animal. (From Leuckart and Nitschc's Diagrams.) 



the aboral side of the disc are whorls of slender, curved, cylindrical 

 appendages, the cirri (Fig. 331), by means of which the Feather-star 

 is enabled to anchor itself temporarily to a rock or a sea-weed. 



On the oral side of the disc the body- wall is soft and flexible, 

 containing only scattered irregular spicules of calcareous matter ; 

 and nearly, but not quite, in the centre of this surface is an opening, 

 the mouth (Fig. 332, mo). From the mouth five very narrow 

 grooves, the ambulacral grooves, radiate outwards towards the bases 

 of the arms, near which they bifurcate, so that ten grooves are 

 formed, one passing along the oral surface of each of the ten 

 arm-branches to its extremity. The anal opening (an) is likewise 

 on the oral surface, being situated on a papilliform elevation in 

 the interspace between two of the radiating canals. 



