446 



ECHINODERMATA ASTEROIDEA 





of physiological experiment, and so Starfish have been favourite 

 " corpora vilia " with many physiologists. 



The light-perceiving function of the eye is easily demon- 

 strated. If a number of Starfish be put into a dark tank 

 which is illuminated only by a narrow beam of light they will 

 be found after an interval to have collected in the space reached 

 by the beam of light.-' If all the median tentacles but one be 



removed this will 

 still be the case ; if, 

 however, they are 

 all removed the 

 Starfish will exhibit 

 indifference to the 

 light. 



If the under sur- 

 face of a Starfish 

 be irritated by an 

 electric shock or a 

 hot needle, or a 

 drop of acid, the 

 tube - feet of the 

 affected area will be 

 strongly retracted, 

 and this irritation 

 will be carried by 

 the pedal nerves to 

 the radial nerve- 

 cord, with the re- 

 sult that finally all 

 the tube-feet in the 

 groove will be retracted and the groove closed by the action of 

 the transverse muscle connecting each ambulacral ossicle with 

 its fellow. If, on the other hand, the back of a Starfish be 

 irritated this may produce a contraction of the tube-feet if the 

 irritation be strong, but this will be followed by active alternate 

 expansions and contractions, in a word, by endeavours to move. 

 Preyer- by suspending a Starfish ventral surface upward, by 



MB 





Fig. 193. — A, longitadinal section of a single eye-pit of 

 Asterias. s.n, Nucleus of supporting cell ; vis.n, niicleus 

 of visual cell ; vis.r, visual rod. B, view of the terminal 

 tentacle showing the eye-pits scattered over it. (After 

 Pfeffer.) 



' Romanes, " Jellyfisli, Starfish, and Sea Urchins, " Intern. Scientific Series, 1885, 

 pp. 320, 321 ; Preyer, " Bewegungen von Stelleriden," Mitth. Zool. Slat. Neapcl, 

 vii. 1886-87, p. 22. = Preyer, loc. cit. p. 49. 



