OLFACTORY SET.E OF THE CLADOCEEA. 287 



not know that they are olfactory setae at all. No experiments 

 have ever been made, nor is it hardly conceivable how experi- 

 ments could be made, to test the matter. Leydig, however, 

 judging from the analogy of these setae to the micro-structure 

 of known organs of smell, came to the conclusion that they also 

 possessed an olfactory function, and his opinion has been 

 adopted practically by all the zoologists who have alluded to 

 the subject. This is to be sure not a very strong position, but 

 it receives support from negative evidence. Thus the seta? 

 cannot be organs of sight, for the animals are provided with 

 evident compound and simple eyes. They cannot be merely 

 organs of touch because of their peculiar structure. It is 

 extremely improbable that they are organs of hearing, because 

 in some of the higher Crustacea (e.g., the crayfish) similar 

 setae occur on the antennules, while much more probable organs 

 of hearing are also present. Lastly they cannot be considered 

 as the seat of the sense of taste owing to their position, which 

 is often far removed from the mouth organs. It appears toler- 

 ably certain, therefore, that unless the setae under consideration 

 minister to some sense unknown to us, they must be olfactory 

 organs. 



Explanation of Plate XIII. 



PiG. 1. Rostrum of Daplinia pulex showing antennule and 

 olfactory setas, etc., X 75. a. Antennule. b. 

 Chitinous beads at bases of olfactory setae, g. 

 Ganglion. 7. Lateral pointed seta. n. Nerve 

 from brain, o. Tuft of nine olfactory setae, t. 

 Nerve threads from olfactory setae to ganglion. 

 ,, 2. An olfactory seta from Leptodora liyalina x 400* 

 ,, 3. ,, „ ,, ,, Simocephalus vetulus X 



600. 3a. Tip of seta. 

 n 4. ,, ,, ,, ,, Eurycercus lamellatus x 



600. 4a. Tip of seta. 

 Homologous parts are 

 lettered as in Fig. 7. 

 ,, 5. ,, ,, ,, , Daphnia pulex x 700. 



,, 6. ,, ,, ,, , Acantholeberis curvirostris 



x 400. 



