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The Olfactory Setm of the Cladocera. 

 By D. J. Scourfield. 



(Bead September 18th, 1896.) 



Plate XIII. 



The antennules, or first pair of antennae, of the Cladocera, 

 although for the most part organs of small size and simple 

 structure, nevertheless exhibit a very considerable range of 

 variation in form in various species, while the differences 

 between these organs in the two sexes of the same species are 

 in nearly all cases well marked, and not infrequently become 

 quite striking. But to whatever extent an antennule may be 

 modified it is always provided with a tuft of minute char- 

 acteristic setae, the so-called olfactory setae. This tuft of setae 

 is so essentially part and parcel of the antennule that it is 

 found to persist even when the latter is altogether absent as a 

 distinct outgrowth, as occurs in the marine genera Podon and 

 Evadne. About two years ago my attention was specially 

 attracted to these peculiar little setae through having acci- 

 dentally noticed that, in many species, the number in each tuft, 

 i.e., on each antennule, was constant. Since that time I have 

 examined, as opportunity offered, a great many specimens from 

 this point of view, and it is proposed in the present paper to 

 bring forward the results so obtained, together with a review 

 of the principal facts hitherto recorded, which, it is almost 

 needless to say, lie scattered in the most diverse publications. 



Notwithstanding many earlier allusions to the setae in ques- 

 tion by writers on the Entomostraca, Leydig was the first to 

 really appreciate their special nature and to give satisfactory 

 descriptions of them. In his " Lehrbuch der Histologic" 

 (1857), "Naturgeschichte der Daphniden " (1860), and a 

 paper " Ueber Geruchs-und Gehororgane der Krebse und 

 I usekten " (" Archiv fur Anatomie," 1860), he not only gave a 

 ^ood account of the histology of these seta?, but also made 

 known the wide distribution of closely similar structures 



