ECHINODERMA. 



149 



Dr. Sluiter* has since put on record his regret that he did not become acquainted 

 with my note, when he still had under his eyes Ophiuroids that were living in his 

 aquaria without the dorsal covering to their disks. The phenomenon seems to be 

 pretty common, and there are a number of interesting biological problems associated 

 with it which should attract naturalists in tropical and subtropical seas. 



Alter some search I lit on a long-armed Ophiuroid with the upper surface of the 

 disk intact ; it was soon easy to see that this was not the Ophiocnida imbricata in 

 which Dr. Sluiter had noticed the phenomenon, and I was inclined to ascribe it to 

 Amp/dura divaricata, but the arms of that species, as described by Ljungman, are 

 much shorter. It appears to be a true, but unnamed, Amphiuran. 



(4.) I beg once more to offer an example of the variability of Echinoderms, and to 

 call attention to the mode of distribution of the spicules on the supermarginal plates 

 of Astropecten hemprichi; the three figures here shown are taken from the three 



specimens found in the bottle to which the late Mr. Sladen affixed the name of 

 Astropecten zebra. In the "Challenger" Report A. zebra occupies the following 

 position in the author's " key " : 



(a.) With small spinelet, on the first four or five plates. 



(a.) With four or five spinelets. A well-developed series of 



pseudo-pedicellarise zebra. 



(b.) With one spine only on the first plate. No pedicellariae . . velitaris. 



Inspection of my photographs will show how little constant is the number of 

 spinelets, and the uselessness of the character as an aid to specific distinction. 



* 'Tijds Nederl. Dierk. Ver.,' v. (1898), p. 300. 



