IX 



PHYLUM ECHINODERMATA 



373 



4. TYPE OF THE CRINOIDEA. 



A Feather-Star. Antedon rosacea. 



General External Features. In the Feather-star (Fig. 301), 

 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 ventral or actinal 

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

 abactinal surface downwards. The five arms are bifurcated at 

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

 the locomotive organs of the animal, their alternate flexions and 

 extensions resulting in a slow movement through the water. On 



FIG. 301. Antedon. Side view of entire animal. (From Leuckart and Nitsche's Diagrams.) 



the dorsal side of the disc are whorls of slender curved cylindrical 

 appendages, the dorsal cirri, by means of which the feather-star is 

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



On the ventral side of the disc the body- wall is soft and flexible; 

 containing only scattered irregular spicules of calcareous matter, 

 and in the centre of this surface is an opening, the mouth (Fig. 

 302, mo.). From the mouth five very narrow grooves, the ambulacra! 

 grooves, radiate outwards towards the bases of the arms, near which 

 they bifurcate, so that ten grooves are formed, one passing along 

 the ventral surface of each of the ten arm-branches to its extremity. 

 The anal opening (an.) is likewise ventral, and is situated on a 

 papilliform elevation in the interspace between two of the radi- 

 ating canals. 



