Agassiz.] 106 [June 16, 



whole abactinal area is expanded to its fullest amount, the arms and 

 interradial part swelling out immensely, become quite flaccid, and the 

 specific gravity of the starfish must, by the amount of water which it 

 has thus taken in, be very nearly equal to that of water. 



This operation I have repeatedly seen performed by starfishes, 

 measuring five or six inches across the arms, and when the starfishes 

 are young they frequently lose their hold, and float about on the 

 surface of the water. Starfishes measuring two and one half and 

 three inches across the arms, are able to float in this manner, and 

 while fishing with the scoop net on the surface of the water for pelagic 

 animals, hardly a single expedition goes by without finding one or two 

 young starfishes, ranging from one eighth of an inch to one and one 

 half inch across the arms swimming freely about. This is not limited 

 to our two species of Asteracanthion. I have observed the same thing 

 in the adult of our common Crilrella when kept in confinement, and 

 young measuring three fourths of an inch across the arms, are fre- 

 quently found swimming about. I had already in' 1864 * called atten- 

 tion to the fact that young Sea-urchins, measuring somewhat less than 

 one fifteenth of an inch in diameter, could be found with the scoop net 

 swimming on the surface, and that the young of our Ophiurans, young 

 Ophiopholis having already five joints on each arm, long after they have 

 lost their plutean appendages, have the same habit of floating at will 

 on the surface by expanding to their fullest extent. The same applies 

 to young Cuvieria one half inch in length. I have never observed any 

 such capacity in the older Ophiurans or Cuvieria. But there is an- 

 other Echinoderm which, in the adult condition, is capable of a kind 

 of swimming, that is the Euryale. In my work on Radiates of Mas- 

 sachusetts Bay, I gave a sketch of an Euryale' in its natural attitude 

 standing on tip toe, as it were, with its disk swelling to the fullest 

 capacity, thus supporting the whole weight upon comparatively small 

 numbers of the slenderest joints of the extremity of the arms, showing 

 how nearly equal to the specific gravity of the water it must be, so that 

 by slightly pushing it and producing a current, it will actually float, 

 and then slowly settle again to the bottom. This will account for the 

 frequency with which our fishermen bring up Astrophyton on their 



1 Agassiz, A. On the Embryology of Echinoderms. From Mem. Am. Acad, 

 IX, p. 9. 



8 Seaside studies in Natural History. Fig, 151, p. 118. Boston, 1865. Marine 

 Animals of Massachusetts Bay. Radiates. By El. and Alex.Agassiz. 



