OLFACTORY SET^ OF THE CLADOCERA. 285 



closely similar olfactory seta? on each joint of the outer branch 

 of the antennule of the crayfish ? (See figures given by Ley dig, 

 " Archiv f. Anat.," 1860, Plate VII., and Haxley, " The Cray- 

 fish," 1880, p. 114). 



As regards the number of olfactory setae present in the males 

 of the Cladocera, I have not been able to examine a sufficiently 

 representative series of specimens to make any generalisations. 

 It seems quite certain, however, that among the Daphnidae the 

 males have the same number of setae as the females, namely, nine. 

 The same is probably true also of the Sididae. The males of 

 the Lynceidae, on the other hand, very often possess twelve 

 olfactory setae on each antennule, although occasionally only 

 nine are present, as in the female. The male of Graptoleberis 

 testudinaria is somewhat anomalous in this respect, as it seems 

 to have eleven setae. 



Before leaving this part of the subject it may be well to 

 call attention to two sources of error which are liable to lead 

 to false results in counting the number of olfactory setae. The 

 first is that some of these delicate setae may be accidentally 

 torn off the antennule, thus apparently reducing the total 

 number. The error arising from this cause can be avoided 

 by counting the number of elongated beads within the 

 antennule. The second source of error is that setae of various 

 descriptions other than olfactory setae are often most intimately 

 associated with the latter. (See Figs. 10, 11 and 12.) The 

 only way to avoid counting these is obviously to make sure 

 that each seta possesses the little terminal pellet and the basal 

 bead of chitin. 



There now remain for consideration a few facts in connection 

 with the arrangement of the olfactory setae on the antennule 

 and the variations in the length of the setae themselves. 



As already remarked the tuft of setae usually originates at 

 or very near to the tip of the antennule, but it sometimes 

 happens that the tuft appears to be shifted nearer to the base 

 of the latter. An extreme case is to be found in Latona setifera, 

 where, owing to an enormous development of the structure 

 homologous w r ith the so-called flagellum in the genera Sida 

 and Diaphanosoma (Daph?iella), the tuft of olfactory seta? 

 seems to originate quite close to the base of the antennule 

 Species of Bosmina also show the olfactory setae nearer the bate 



