ALTERNATE GENERATIONS. 255 
more ; and at this time they prefer the sunniest 
exposures, and like to bask in the light and 
warmth. They assume every variety of attitude, 
but move always by the regular contraction and 
expansion of the disk, which rises and falls with 
rhythmical alternations, the average number of 
these movements being from twelve to fifteen in 
aminute. There can be no doubt that they 
perceive what is going on about them, and are 
very sensitive to changes in the state of the 
atmosphere ; for, as soon as the surface of the 
water is ruffled, or the sky becomes overcast, 
they sink into deeper water, and vanish out of 
sight. When approached with a dip-net, it is 
evident, from the acceleration of their move- 
ments, that they are attempting to escape. 
At the spawning season, toward the end of 
July or the beginning of August, they gather 
again in close clusters. At this period I have 
seen them at Nahant in large shoals, covering a 
space of fifty feet or more, and packed so closely 
in one unbroken mass that an oar could not be 
thrust between them without injuring many. So 
deep was the phalanx that I could not ascertain 
how far it extended below the surface of the 
water, and those in the uppermost layer were 
partially forced out of the water by the pressure 
of those below. 
It is not strange that the relation between the 
