76 DISCOPHOR^. Part III. 



paler color, than during the month of July, when they complete their entire develop- 

 ment, at the approach of the spawning season. 



When they first make their appearance, early in the spring, these Medusae may 

 be seen, as the sun rises higher above the horizon, floating in immense numbers 

 near the surface of the water, as long as the sky is clear, the sun shines brightly, 

 and the surface of the water remains smooth; though, at that time, they do not 

 seem to seek the places most exposed to a glaring sun, but, on the contrary, 

 appear more frequently about sheltered places, in the neighborhood of wharves, or 

 near prominent rocks. They are, at this time, gregarious, but evenly scattered 

 through the water, and nowhere crowding upon one another. As they grow larger 

 they scatter more, and are found, at a greater distance from the shore, sometimes far 

 apart one from the other, and evidently preferring the sunniest exposures. They 

 may then be seen floating in every attitude, moving to and fro by the rythmical 

 contraction and expansion of their disk, which, as they advance, is always turned 

 in the direction of the progress. These contractions and expansions are as regular 

 as respiratory movements ; their rhythm presents slight variations only, larger speci- 

 mens, however, contracting and expanding at somewhat longer intervals than smaller 

 ones. The average number of these movements is from twelve to fifteen in a 

 minute. There can be no doubt that these animals perceive what is going on 

 about them, and that they are very sensitive to changes in the condition of the 

 atmosphere. As soon as the surface of the water begins to be ruffled, ever so 

 slightly, by the unequal pressure of the atmosphere, or the sky becomes cloudy 

 or overcast, they sink into deeper water and vanish out of sight. Even accidental 

 disturbances are perceived by them, for when approached, however carefully, the 

 change of their course, or the unusual rapidity with which they sink, shows plainly 

 that they are making the utmost efforts to escape, though their ability to do so 

 is very limited. But under such circumstances their rythmical movements are 

 plainly accelerated, their contractions more powerful, in consequence of which their 

 increased specific gravity may accelerate their progression or facilitate their descent 

 into deeper water. 



At the time of spawning, towards the end of July or the beginning of August, 

 they may be seen gathering again and clustering nearer together. That at this 

 time they seek one another is imquestionable. I witnessed once, in front of my 

 house at Nahaut, a shoal of them, which was evidently in the act of spawning. 

 It could be seen from the shore, at about half a mile's distance. Myriads of speci- 

 mens had clustered together so closely that they formed an unbroken mass, between 

 which an oar could not be thrust without hitting many at one blow. They were in 

 such a deep phalanx, that it was impossible to ascertain how far below the sur- 

 face they extended, while those in the uppermost layer were partially forced out of 



