156 Papers from the Marine Biolos^ical Laboratory at Tortugas. 



surrounding waters. On the oilur liaiul. 1 Ikivc observed that when they 

 first appear they come tip from deep water, and their disappearance can be 

 seen to be dtie to tlieir sinking from the stirfacc into greater depths. In 

 1907 I first found them outside of Nassau liarlior far below the surface; 

 tlay after day tliey were found nearer the surface, until the great swarm aji- 

 pearcd, after wliicli they again settletl into deeper water. 



The cause of their sudden disappearance may be observed in the labora- 

 tory as well as in nature. Tlie sexual products are shed during the swarming 

 and after the gonads are emptied the medusa; settle to the bottom and soon 

 thereafter begin to disintegrate, as may readily be seen in an aquarium. It 

 can also be observed that the same thing takes place in nature, where many 

 dead or dying medusa; may be found over the bottom in shallow water after 

 the swarm has disappeared. The enormous numbers of the swarm and its 

 limited duration, as well as the fact that the medusje come up from deep 

 water, all indicate that they must be formed and set free in the immediate 

 vicinity of the place where they a])iH'ar at the surface. 



Movement to tlie surface is an active and not a passive process; the 

 medus;t and their eggs are heavier than water, and they settle to the bottom 

 when undisturbed or when movement ceases. The movements to or from 

 the surface are not correlateil with the intensity of light, as is the case in so 

 many pelagic organisms, for during the swarming these medusa; are at the 

 surface at all hours of the day and, so far as I could observe, of the night 

 also. Furthemiore, they appear at tlie surface at no other time than the 

 swarming period. It is highly probable that the movement to the surface 

 is correlated with tlie ripening of the sexual products ; and it is certain that 

 the swarming is of importance to the species, since the ripe males and 

 females are thus brought near together at the time when the sex products 

 are ready to be shed. The collection of these mednsK into windrows is of 

 great advantage in insuring the fertilization of the eggs, but it is probably 

 not brought about by the activities of the animals, other than those which 

 serve to keep them at the surface of the water. Frobably the winds and 

 waves gather them into the windrows. 



Movements. — The normal movements of Linerges are of two kinds : ( i ) 

 the pulsations which drive the animal forward in swimming, and (2) rhyth- 

 mical llattening of the bell, by bringing together opposite sides, so that the 

 opening into the bell is elongated first in one direction, then in another. 

 These latter movements are especially prominent when the medusae are con- 

 fined within a small space, are much irritated, or when lying on the bottom. 

 This flattening of the bell may be violent, in which case the opening into 

 the bell becomes slit-like, or it may be weak, in which case the bell opening 

 is quadrangular or triangular in shape. When violent, the directions of 

 flattening are alternately almost at right angles, but not quite. When viewetl 



