24 LEON J. COLE 



appear to evince ''a sort of preference for certain arms;" never- 

 theless the general conclusion appears to be that no ray has 

 especial value as a director. This, like many of Bohn's con- 

 clusions, one wishes might be based on a more extended series 

 of observations. 



E. C. and A. Agassiz ('65) state that ''Cribrella [Cribrella 

 oculata = Henricia sanguinolenta] moves usually with two of 

 the arms turned backward, an*d the three others advanced to- 

 gether, the two posterior ones being sometimes brought so close 

 to each other as to touch for their whole length." They make 

 no statement, however, as to whether it is usually the same 

 arms which take these positions. 



1. Preyer's experiments on starfishes 



Preyer ('86-7) appears to be the only person who has pre- 

 viously investigated this question in anything like a statistical 

 way, and although he concluded negatively as to the preferen- 

 tial use of any particular arm or arms as directors, his results 

 are worthy of examination in some detail. His method of experi- 

 mentation was careful and ingenious, and one feels inclined to 

 place confidence in what he speaks of as these 'sehr zeitraubenden 

 Versuchsreihen.' He laid the animals on the top of a glass 

 support, which was hemispherical above, and so arranged that 

 while the greater parts of the rays of the specimen were immersed 

 in the water, their basal part and the central disc were exposed 

 to the air. The support was filled with water except for an air 

 bubble at its top, by means of which it could be accurately 

 leveled. Finally the specimens were oriented with a given ray 

 to the north, east, south and west in successive trials, in order 

 to neutralize the possible interference of any inequalities in the 

 surroundings. The starfishes and brittle stars used naturally 

 tended to crawl down one side of the glass support so as to im- 

 merse themselves completely in the water, and Preyer recorded 

 what ray was in advance in this movement. He numbered the 

 rays 1, 2, 3, 4 and 5, beginning with the one which hasthemadre- 

 porite at the left of its base and counting around clockwise, 

 these numbers therefore corresponding respectively to raj's a, 



