. Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage. Zoogeography -- Geographical distribution; Animal ecology. 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
. Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage. Zoogeography -- Geographical distribution; Animal ecology. 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 effectiv
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