382 Biology in America 



bacteria in tlic spore stage can survive boiling. Some fish 

 even may witJistand a temporatuie of 128°F. Many forms 

 of aquatic life however can api)arently only survive the win- 

 ter as eggs or other resting bwlies, which settle to the bottom, 

 there remaining quiescent until the advent of the spring. 

 The eggs of some crustaceans apparently require to be frozen, 

 whiU; others apparently inust be dried as well as frozen in 

 order to hatch. Many of them doubtless can survive long 

 periods of drouth and cold. Those of Estheria have been 

 hatclied after being kept dry for nine years. 



Some aquatic animals survive unfavorable conditions by 

 surrounding themselves with a shell or cyst and lying dor- 

 mant for a time. The deeper waters in some lakes are ap- 

 parently entirely free from oxygen in summer. At tliis time 

 Professors Birge and Juday have found in certain Wisconsin 

 lakes a Cyclops, which surrounds itself with a gelatinous 

 shell, and goes to sleep until the cooler weather in the fall 

 causes the surface water to gradually settle to the bottom, 

 carrying with it the oxygen which it has absorbed from the 

 air; whereupon, the Cyclops wakes up, and throwing off its 

 sleeping jacket, resumes the ''strenuous life" once more. 



Many crustaceans, rotifers and worms lay two kinds of 

 eggs — a thin-shelled "summer" c^^ which develops quickly 

 without fertilization, and a thick-shelled "winter" egg, which 

 after fertilization passes the winter at the bottom of pond 

 or pool to hatch in the succeeding spring. Fresh water 

 sponges and Bryozoa fonn resting bodies known as "stato- 

 blasts, " which over-winter in a resting condition, resuming 

 active growth in the spring. Flowering aquatic plants such 

 as the ditch grass, the crowfoot or the water-cress, may like 

 land plants, live for either one or more seasons. In the for- 

 mer case the life of the species is continued by seeds or re- 

 sistant buds, which, like the "winter eggs" of various spe- 

 cies of animals, live over winter at the bottom of pool or pond 

 to become active when the face of spring smiles again upon 

 the waters. In plants which live for more than one season 

 much of the plant dies in the fall, leaving mainly the under- 

 ground parts persisting over winter, from which the aquatic 

 and aerial stems and leaves are renewed the succeeding year. 

 Some plants however such as the hornwort (Ceratophyllum) 

 and the water- weed (Elodea) may retain their leaves through- 

 out the winter beneath the ice, and aid in furnishing oxygen 

 to support the animal inhabitants of the water. Plants and 

 animals living in temporary' pools can only persist through 

 the formation of resting bodies of some sort — seeds, eggs, 

 etc., which can withstand a prolonged period of drying. 



