﻿Contributions to the Natural History of the Acalephje of North America. 



By L. AGASSIZ. 



PART II. — On the Beroid Medusa of the Shores of Massachusetts, in their Perfect Stale of Development. 



{Communicated to the Academy, May 8th and May 29th, 1849.) 



PLEUROBRACHIA. 



The character of the Beroid Medusae is entirely different from that of the Dis- 

 cophorae. Both their form and organs of locomotion give them a different appearance. 

 The common Discoid Medusae, setting aside the various modifications arising from 

 marked peculiarities of their outline, move like an umbrella, which, alternately opening 

 and shutting, would make its way under water by means of such movements. It is by 

 contraction of the body itself, — of its mass, or rather of the muscles which pervade 

 that mass, — that motion is produced in those animals. Not so in the Beroid Medusae, 

 where the whole body, more or less spherical or ovate, compact or split at one end, 

 is kept swimming by the flapping of innumerable small oars, arranged in vertical rows, 

 like the ribs of an orange, upon the outer surface, along which horizontal combs of little 

 fringes move with extraordinary rapidity, forming a sort of revolving wheel. These rows 

 are generally eight in number, extending from one point to the opposite side, like the 

 meridians of an artificial globe. But owing to the inequalities in the motions of their 

 vertical flappers, and their almost circular arrangement around the more or less spherical 

 body, these animals have a somewhat rotatory motion, unless the oars move on all sides 

 with perfect steadiness and uniformity. 



There can be scarcely any thing more beautiful to behold, than such a living trans- 

 parent sphere sailing through the water, running one way or another, now slowly revolv- 

 ing upon itself, then assuming a straight course, or retrograding, advancing, and moving 

 sideways in all directions with equal precision and rapidity, then stopping to pause, for 



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