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The Ephippium of Bosmina. 



By D. J. ScoLRFiELD, F.R.M.S. 

 {Read January 18f/i, 1901). 



Plate 6. 



The production of winter or resting eggs in the genus 

 Bosmina has been referred to by various writers on the 

 Entomostraca, but I do not think that any description of the 

 egg's protective covering, which corresponds, of course, to the 

 ephippium of the Daphnidae, has hitherto been published. As 

 this structure exhibits one feature at least which distinguishes 

 it from the homologous productions of all other forms of the 

 Cladocera, it appears worth while to bring forward the present 

 short paper on the subject. 



If we examine the recently thrown-off resting egg of Bosmina 

 longirostris enclosed in its protecting case (Eig. 1), we shall see at 

 once that the latter is only a portion of the carapace of the 

 mother. The particular part of the shell which has been utilised 

 for the purpose evidently consists of the valves (as distinguished 

 from the head-shield), with the exception of a rather large piece 

 of their ventral margins. The ventral margins have not wholly 

 disappeared, for the characteristic shell-spines at the posterior 

 ventral angle are still present. At first sight it does not seem 

 that the portion of the shell now enclosing the resting egg has 

 been specially modified. The ordinary faint hexagonal markings 

 on the surface of the valves are quite apparent, and the valves 

 themselves are as transparent as when forming part of the coat of 

 the living animal. Towards the back there is, it is true, a some- 

 what darker tinge than usual, but this is not very noticeable, 

 and taken by itself would scarcely suggest special modification. 

 Looking more closely at the structure, however, it will be seen 

 that at the back — i.e., along the line representing the dorsal 



