486 



ECHINODERMATA OPHIUROIDEA 



CHAP. 



traverses the outermost flange of 



functionless remnant of the 

 retractor muscles of the 

 stomach of Asteroidea. In 

 the young Asteroid there is 

 a similar sheet of membrane, 

 which later becomes resolved 

 into the ten retractor bands. 



The simple structure of the 

 alimentary canal appears to 

 be correlated with the ex- 

 ceedingly simple character of 

 the food. Ophiothrix feeds 

 on the most superficial layer 

 of mud at the bottom of the 

 sea. This deposit consists 

 partly of microscopic Algae 

 and partly of decaying organic 

 matter, and is much more 

 easily disposed of than the 

 living animals on which the 

 Starfish preys. The food is 

 shovelled into the mouth by 

 the first two or " buccal " 

 pairs of tube-feet in each ray. 



The water -vascular 

 system has undergone a most 

 interesting set of modifica- 

 tions, which can be explained 

 by noticing the fact that the 

 tube-feet have almost, if not 

 quite, lost their locomotor 

 function and are now used as 

 tactile organs. The ampulla, 

 or swollen inner end of the 

 tube-foot, has disappeared, 

 and the upper end of the 

 organ is directly connected 

 with the radial canal by means 

 of a curved canal, which 

 the vertebra, appearing on its 





