352 



ANIMALS IN INLAND WATERS 



The resting eggs of these Crustacea are so enduring that they can 

 remain dry for years without losing their ability to develop ; specimens 

 of mud from east Africa which had been dried for fourteen or fifteen 

 years produced larvae of Euphyllopoda. This also explains the fact 

 that these crustaceans are often absent from their usual habitat for 

 a number of years and then suddenly appear again in great numbers 

 in the same place. The ability to retreat into a habitat whose condi- 

 tions are so severely selective has probably made it possible for these 

 primitive phyllopods to continue to survive while their relatives suc- 

 cumbed in the competition with more highly evolved forms, a familiar 

 phenomenon among relicts. 



Cladocera also produce eggs which are not injured by drying, the 

 so-called resting eggs which are surrounded by ephippia (Fig. 103). 



These eggs must be fertilized and 

 consequently are produced only when 

 the male forms are present; the eggs 

 which develop parthenogenetically 

 without fertilization have no such re- 

 sistance to drying. The effectiveness 

 of the protection rendered by the 

 covering is shown by the fact that 

 they pass unharmed through the ali- 

 mentary canal of fishes, and even 

 withstand formaldehyde. Rhabdocoel 

 flatworms also occasionally produce hard-shelled resting eggs, while 

 vernal planarians such as PL velata enclose themselves, as do certain 

 copepocls, in hardened slime capsules. The fertilized eggs of rotifers arc 

 also protected by hard shells. In Daphnia magna and D. pulex rest- 

 ing eggs may appear shortly after the first emergence in spring,™ 

 in the rotifer Hydatina senta, as early as the second generation. Ostra- 

 cods, which are also common in small puddles, often have ridges on 

 the ventral border of one valve and thus provide for an effective 

 closing of the shell. 



Short periods of development and great fertility are often char- 

 acteristics which adapt animals to life in transient waters. Many 

 species of Branchipus among the euphyllopods become capable of re- 

 producing in one week, and even the large gill-footed Apus is sexually 

 mature after 14 days, at a length of 1.5 cm. Both appear in a single 

 annual generation; but many of them lay 300-400 eggs daily for 

 weeks, 80 and the total number of eggs produced by a female of 

 Tanymastix lacunae has been estimated at 17,000. 81 Daphnia magna 

 becomes mature in five to six days under favorable conditions of 



Fig. 103. — Ephippium of a copepod, 



Daphnia pulex, enclosing two winter 



eggs. After Lauterborn. 



