1 882.] Zoology. 815 



cavity, and is the result of a secondary folding of the common 

 shell envelop. At the close of the development in the egg, this 

 "fail" lies between the valves of the shell, curved beneath like 

 the tail of a frightened dog, although the frequent motions of the 

 post-abdomen are not a little hindered thereby. 



On its escape from the egg, the animal swims freely, and soon 

 kicks this pliant appendage backward and upward till it assumes 

 a direction parallel to the long axis of the body, and then very 

 soon its unequal growth causes this tail to be somewhat elevated. 

 The appendage probably serves as a support for the cast off skin 

 in the molt, so that it cannot fall down upon the post-abdomen 

 and then be broken off before that portion of the shell forming 

 the inner covering of the brood cavity can be successfully molted 

 —a danger especially incident to long forms with narrow brood 

 cavities, and to young animals in which the shell is tender. (It 

 may be for this reason that males, in which the part correspond- 

 ing to the brood cavity is very narrow, and young females, have 

 this spine, while adult females do not, for, as is well known, the 

 males of all this section of the genus are spined through life.) 

 Successive moltings increase the size of the animal, but the spine 

 remains and increases correspondingly, giving the animal a very 

 different appearance from the parent, which was not only of an 

 entirely different form but totally without the spine. 



Finally the young female produces eggs parthenogenetically, 

 and is, therefore, according to our customary notions, an adult. 

 We have here, therefore, a case of heterogenesis. Under circum- 

 stances where food is not sufficiently abundant, it seems certain 

 that the above-mentioned state is the final one, and that the ani- 

 mal does not reach that condition which we name Daphnia magna, 

 but remains in a stage which has received a different specific 



The same process has since been observed in the case of Daph- 

 nui pulcx,\n Minnesota. Some of the so-called varieties are but age- 

 forms. There is in each species what maybe called a post- imago 

 form, which is only assumed under favoring conditions. With- 

 out going into the synonymy of this genus, which will bear a 

 revision in view of this and similar facts, we may safely say that 

 m the Daphnidae we find heterogenesis almost a rule, at least in 

 the genus Daphnia. 1 We may add that every possible provision 

 tor the reproduction of these animals seems to be provided. 

 U) They are very prolific ; (2) reproduce both sexually and par- 

 thenogenetically ; (3) resist great extremes of temperature ; (4) 

 accommodate themselves to great alterations in the purity of the 

 water ; (5) the winter eggs are provided with a horny covering 

 01 '/'V/'. 1 u, whi. h permits them to be dried in a mass of mud 

 °r frozen in a cake of ice without destroying their vitality ; (6) 



1 See Birge, Notes on Cladocera, Madison, Hate II, Fig. 6. 



