262 Adjustment to Conditio?! s of Aquatic Life 



tion. Here competition for food and standing room is 

 most severe. And here are made some of the most 

 remarkable shifts for maintaining "a place in the sun.'' 



Encystment — The shifts which we are here to consider 

 are those made in avoidance of the struggle — shifts 

 which have to do with the tiding over of unfavorable 

 seasons by withdrawal from activity. This means 

 encystment or encasement of some sort or in some 

 degree.* The living substance secretes about itself 

 some sort of a protective layer, and, enclosed within it, 

 ceases from all its ordinary functions. 



This is the most familiar to us in the reproductive 

 bodies of plants and animals; in the zygospores of 

 Spirogyra and desmids and other conjugates; in the 

 fruiting bodies of the stoneworts; in the seeds of the 

 higher plants ; and in the over-wintering eggs of many 

 animals. Most remarkable perhaps is the brief seasonal 

 activity of forms that inhabit temporary pools. Such 

 Branchipods as Chirocephalus (see fig. 90 on p. 184) 

 Estheria and Apus, appear in early spring in pools 

 formed from melting snow. They run a brief course of 

 a few weeks of activity, lay their eggs and disappear to 

 be seen no more until the snows melt again. Their 

 eggs being resistant to both drying and freezing, are 

 able to await the return of favorable conditions for 

 growth. The eggs of Estheria have been placed in 

 water and hatched after being kept dry for nine years. 



But it is not alone reproductive bodies that thus tide 

 over unfavorable periods. The flat worm, Planar ia 

 velata, divides itself into pieces which encyst in a layer 

 of slime and thus await the return of conditions favor- 

 able for growth. The copepod, Cyclops bicuspidatus, 

 according to Birge and Juday (09) spends the summer 

 in a sort of cocoon composed of mud and other bottom 

 materials rather firmly cemented together about its 



