286 D. J. SCOURFIELD ON THE 



than the tip of the antennule, and in many males somewhat 

 similar alterations in the position of the tuft of olfactory setae 

 occur. 



The variation in position is not, however, confined to the tuft 

 as a whole, but also occurs among the individual setae. In 

 most species the setae have their bases close together, but cases 

 are also to be found in which one or more setre are separated 

 somewhat from their fellows. Good examples of such an 

 arrangement are exhibited by Acroperus harpce and Alonopsis 

 elongata where one of the nine olfactory setae is shifted back 

 from the main group about one-third the length of the anten- 

 nule. In yet another case, Chydorus latus, two of the seta? are 

 moved back a little from the remaining seven. 



Differences of length among the setae of the same tuft are 

 most numerous in the Lyncodaplmidae and Lynceidee. Some 

 of these are very characteristic, and combined with differences 

 of position can often be relied upon for the determination of 

 species. For instance Alona quadrangularis and A. affinis are 

 so much alike that they have doubtless been mistaken one for 

 the other times without number. Yet it is quite easy to 

 distinguish them merely by looking at the arrangement and 

 comparative lengths of the setae on the antennules. (Compare 

 Figs. 8 and 9). It must be understood that these remarks 

 apply only to the same sex, for the two sexes of the same 

 species often differ in this respect very considerably. Kurz, I 

 believe, was the first to bring forward a case of this kind, for 

 in his paper " Ueber limicole Cladoceren " (" Zeit. f. Wiss. 

 Zool.," Supplement, Vol. xxx., 1878) he showed that the 

 positions of two seta? much longer than the rest are arranged 

 in quite a characteristic way in the three species Ilyocryptus 

 sordidus, I. agilis, and I. acutifrons. 



Many other examples might be given, but enough has been 

 said to show that a careful study of even minute details of 

 structure is not usually thrown away even from the merely 

 systematic point of view. 



In the course of the preceding remarks the term " olfactory 

 setae " has been used quite freely, and it seems only right that 

 in concluding this paper a word or two should be devoted to 

 the question, How do we know that these setae are organs of 

 smell ? Unfortunately it must be admitted that we really do 



