412 



PELMATOZOA. 



as it is in all the ordinary Crinoids ; but in some Feather- stars 

 (species of Actinometrd) it may be quite excentric. The anus (an) 

 is usually supported on a tubular projection, and is excentric in 

 position. The arms of Comatula rosacea exhibit on their ventral 

 surface a deep furrow or groove, the elevated margins of which 

 are cut out into minute crescentic respiratory leaves, at the base of 

 each of which is a group of three tentacles, connected with a cavity 



in the interior of the respiratory leaf, 

 and communicating by a common 

 trunk with the radiating water-vessel. 

 The floor of the ambulacral furrows 

 is ciliated, and underneath each runs 

 a radiating water-vessel, together with 

 a blood-vascular trunk, and a nerve- 

 band which is in intimate relation 

 with the ambulacral epithelium. In 

 the centre of the arm, between the 

 calcareous skeleton and the water- 

 vessel, are three tubular prolongations 

 of the body-cavity. The middle and 

 largest one of these (fig. 287, ov) con- 

 tains one of the generative glands ; 

 while the upper and lower (the " sub- 

 tentacular " and " cceliac " canals) are 

 much smaller, and permit of a circula- 

 tion of water derived from the body- 

 cavity. The slender lateral " pinnules " 

 carried by the arms, as regards their 

 internal anatomy, have precisely the 

 structure of the arms themselves. 



The ciliated grooves on the ventral 

 aspect of the arms are continued over 

 the upper surface of the disc to reach 

 the subcentrally or excentrically placed 

 mouth ; and the animal feeds upon 

 the minute organisms conveyed to the 

 mouth by the water-currents set up 

 along these grooves. The mouth opens into a spirally coiled ali- 

 mentary tube, which forms most of the so-called " visceral mass," 

 and is wholly contained within the calyx, no diverticula from it 

 extending into the arms. 



The water-vascular system consists of a circumoral ring, and of 

 the radiating water-vessels, which run along the brachial furrows. 

 These give off the tentacles, which are destitute of suckers, and 

 are essentially respiratory in function. The circular ring communi- 



Fig. 287. — Cross-section of a pinnule 

 of the Arctic Feather-star, magnified 

 seventy -five times. (From Ludwig, 

 after P. H. Carpenter.) pj, Calcareous 

 skeleton, containing the axial nerve- 

 cord (#) ; ag, Ambulacral groove ; n, 

 Radial nerve ; b, Radial blood-vessel ; 

 iv, Ambulacral vessel ; T, Tentacle ; 

 ov, Ovary, with the subtentacular canal 

 above, and the cceliac canal below ; 

 gv, Genital blood-vessel. 



