450 Entomostraca (Water-Fleas). 



doubted instances of Parthenogenesis* occur. It is certain 

 that with many species one copulation is sufficient to impreg- 

 nate the female for life. And not only this, but that the young 

 which she produces are likewise fecundated and capable, with- 

 out further impregnation, of continuing the species for many 

 successive generations. In Chyclorus sphoericus (a common 

 form), Jurine observed the process through fifteen generations, 

 and through nine in Alona guadrangularis. Dr. Baird has fol- 

 lowed up the successive generations in Daphnioe Pulex as far as 

 the fourth in the Daphnia born from the ordinary ova, and as 

 far as the third in those born from ephippial eggs. These 

 ephippia, or ' ' winter eggs" require, probably, a few words of 

 explanation. They are, in fact, ova covered with envelopes of 

 more than usual hardness and thickness, and thus enabled to 

 withstand an excess of cold, which would surely prove fatal to 

 the parent. They are borne (in the Daphnice) near the back of 

 the shell in a considerable open space termed the matrix. (Fig. 

 5 represents Daphnia rotunda bearing an ephippium.) Here 

 they may be seen in the form of a dense mass of hexagonal 

 cells, through which appear one or two oval bodies, each of 

 which contains an ovum covered with a shell. The ephippium 

 with its enclosed eggs is set free at the fifth moulting of the 

 animal, and floats on the water until spring, when the young- 

 are called into active life by the rays of the ' ' world-reviving 

 sun." The authors of the Micrographic Dictionary, however, 

 reject this theory of the use of the ephippia, saying : — " The 

 formation of this coat can scarcely have any relation to tempera- 

 ture, either from its structure, or from its requirement in an 

 organism which has no heat to retain. Its presence would be 

 perfectly intelligible, however, as a means of protection from 

 evaporation when the pools become dry ; and for this purpose 

 its structure is well adapted. It might also afford a protection 

 against the attacks of predatory animals, many of which could 

 easily devour an ovarian ovum, while they could not break 

 through the horny cases of the winter ova ; and these winter 

 ova are only formed when the ova are not to be hatched soon 

 after extrusion from the parent." It is somewhat startling to 

 read in a scientific work of animals which " have no heat to • 

 retain." Why, even a block of ice has heat in it, and although 

 Entomostraca are what we call " cold-blooded" animals they 

 are only relatively so, and cannot bear more than a moderate 

 decrease of temperature with impunity. If these winter-eggs 

 are meant chiefly as a protection against drought and not 

 against cold, it seems strange that they should not be most 



* For further information on this subject sec Owen's Lectures on Partheuo- 

 genesis, and Dallas's Translation of Von Siebold on A True Parthenogenesis in 

 the Honey Bee, and Silkworm Moth } — the last a most interesting work. 



