338 



Birge — The Crustacea of the Plankton. 



and the head-crest was fairly well grown. On the 22d, a very few 

 of the first generation born in the spring, had laid eggs. On 

 May 12th males were first seen, but 175 females were counted with- 

 out finding any males. On June 3d, it was noted that many of 

 the individuals which had lived over winter were affected by a 

 microsporidial disease, and the young in the brood sacs were 

 attacked and killed by fungus. They were also attacked by 

 bacterial diseases. At this time, or a little earlier, the old 

 individuals were settling into the lower strata of the water, 

 and on the 6th of June nearly all were gone. 



In 1896 the development of the species was in general parallel 

 to that of 1395, but was some two weeks earlier, owing partly 

 to the more rapid warming of the water and partly to the fact 

 that the temperature of the water in winter was slightly higher, 

 and the animals emerged from the winter life in a more ad- 

 vanced condition of development. In each season the surface 

 water had reached an average temperature of 15° C, when the 

 marked rise in numbers occurred. 



The summer numbers of this species appear from the follow- 

 ing table: 



Table XVII. — D. hyalina — Average numbers, June — December, stated 

 in thousands per square meter. 





1894. 



1895. 



1896. 





No 



Obs. 



19.8 



13.3 



16.6 



60.7 



No obs. 



148.4 



207.6 



252.5 



183.1 



No obs. 



121.5 



(49.0) 



319.2 



135.6 



139.9 



275.3} 



273.0 



252.8 



202.8 



201.6 



180.5 



76.6 



56.2 



48.2 



35.0 



44.6 



55.6 



June 16-30 



211.1 



July 1-15 



319.0 



July 16-31 



65.5 



August 1-15 



95.2 



August 16-31 



60.9 





120.4 



September 16-30 



192.5 



October 1-16 



228.0 



October 16-31 



511.5 



November 1-15 



314.6 



November 16-30 



266.0 



December 1-15 



182.8 



December 16-31 



138.9 







