284 



J. SCOURFIELD ON THE 



souvent 7 ou 8." Limiting our attention to the females, it 

 will be found, however, that not only is the number of seta3 

 constant for each species and genus, but that it is also charac- 

 teristic of whole families and groups of families. In fact in 

 the whole range of the Claclocera there are, among the females, 

 only three variations in the number of setae forming each tuft. 

 It may be useful to summarise the facts as follows: — 



The olfactory tuft on each antennule in the females of 



Polyphemidse, 



e.g., Polyphemus, By thot replies, Podon 

 Evadne, 



■1 



consists of 5 seta?, 



9 



Holopedidse, 



e g., Holopedium, 



Sididse, 



e.g., Sida, Diaphanosoma, Latona, 

 Daphnidse, 



e.g., Vaphnia, Simocephalus, Cerioda 

 phrria, Scaplioleberis, Moiua, 

 Boiminidae, 



e.g , Bosmina, 

 Lyncodaphnidae, 



e.g., Macrothrix, Drepanothrix, Aean- > 

 tholeberis, Flyocryptus, Streblocerus, 

 tynceidse, 



e.g., Eurycercus, Acroperus Campto- 



cercus, Alonopris, Leydigia, Grapto- 



Irlrris, Alona, AloneUa, Pleuroxus, 



Earporhynchus, Ghydorus, Monospilus, 



Leptodoiidee, 



e.g., Leptodora, J 



The presence of a generic name in the foregoing statement 

 indicates that at least one species of the genus has been 

 examined from the point of view of the number of olfactory 



It will be recognised at once that the possession of nine olfac- 

 tory setffl on each antennule is almost typical for the whole of 

 the Cladocera, the two families showing variations from this 

 number, Polyphemidse and Holopedidae, being very poor in 

 species. Is it a mere accident that this is also the number of 



