ALTEKNATE 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 

 a minute. 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 



