THE ORGANIZATION OF DAPIINIA PULEX, 



(Concluded.) 



By Thomas Tate, of Bradford, Yoeks. 



The two ephippial eggs are transferred to the dorsal cavity 

 in the usual manner ; become opaque, of a greenish hue, and then 

 development is arrested. Each ovum acquires a horny sheath, 

 and both, on quitting the parent, are enclosed in a single 

 saddle-shaped chitinous bivalved case or ephippium." These 

 envelopes are modifications of the dorsal portion of the carapace, the 

 inner wall forming the sheath of each egg, and the outer, the 

 ephippium. Between these two walls the new carapace arises, and 

 the ephippium, .with its enclosed eggs, is shed at the next moult, when 

 the discarded carapace appears with a bit snipt out of its dorsal 

 portion. Ephippial eggs are met with all the year round, not in 

 autumn only, as the text-books affirm. Agamic eggs and offspring 

 are highly sensitive to temperature or drought, and but for the 

 ephippials the germs would rapidly become extinct. 



The reproduction of Baphniadce is so often inaccurately associated 

 with other modes of evolution, that it may be worth while briefly to 

 review one or two of these statements. It is not unfrequently cited 

 as a case of " alternation of generations." In the sense in which 

 Steenstrup (3) uses the term, we have an example in the Salp<s — free 

 and chained. The free Sal2M produces viviparously a " concatenate " 

 SaVpa^ along a tube, from the surface of which two parallel rows of 

 buds arise, which, when mature, float away in living chains. Each 

 bud develops an ovum and sperm-cell, the offspring of which is a free 

 SaVpa, Each free Salpa is a "nurse" for chained Salpce, just as 

 T(Enia solium is a " nurse " for Cysticercus cellulose. Wagner (4) finds 

 that the larvae of certain gnats, allied to Cecidomyia, do not always 

 produce perfect insects, but give birth to other larvae, which undergo 

 metamorphoses of the usual character, and eventually become gnats. 

 So again among Hydrozoa : the fixed fissiparous Hydra tuba evolves 

 the free swimming oviparous Chrysaora Jiysoscella, a medusa, the 

 egg of which in turn, reproduces the Hydra tuha. 



To " alternation of generations," as exemplified in these cases, the 

 reproduction of Daphnics presents no comparable phenomena. In 



N. S., Vol. i.— May, 1876. 



(3) Publications of Ray Society, vol. 4, 1845. 

 (4) Zeit. fur Wiss. Zool. 1863 ; compare " The Origin of Species," p. 317 (6th edition). 



