58 MARINE ANIMALS OF MASSACHUSETTS BAY. 



were published in the Transactions of the St. Petersburg Academy. 

 Iii the year 1848 Professor Agassiz read a paper upon one of the 

 species of this genus belonging to our coast, before the American 

 Academy, in which he called it Rhacostoma, not being aware 

 that it had already received a name, and gave some account of 

 its extraordinary phosphorescent properties. The name Rhacos- 

 toma must of course yield to that of Zygodactyla, which has a 

 prior claim. 



The average size of this Jelly-fish when full grown is from 

 seven to eight inches in diameter ; sometimes it may measure 



Vlg. 74. 



even ten or eleven, but this is rather rare. The light-violet col- 

 ored disk is exceedingly delicate and transparent, its edge being 

 fringed with long fibrous tentacles, tinged with darker violet at 

 their point of juncture with the disk, and hanging down a yard 

 and more when fully extended, though they vary in length ac- 

 cording to the size of the specimen, and, in consequence of their 

 contractile power, may seem much shorter at some moments than 

 at others. The radiating tubes in this Jelly-fish are exceedingly 



Fig. 74. Zygodactyla seen from above. 



