THE OLFACTQRY SKT.fi OF THE CLADOCERA. 2S1 



among the Crustacea, and in the last-named paper at least 

 considered them to be probable organs of smell. Claus, in a 

 paper " Ueber die blassen Kolben nnd Cylinder an den 

 Antennen dor Copepoden nnd Ostracoden " (" Naturwissen- 

 schaftliche Zeitschrift," 1860), showed that similar setae are 

 present on the antenna? of Copepods and Ostracods, and in 

 various more recent papers he has incidentally added to our 

 knowledge of the subject. Weismann gave a good drawing of 

 a single seta in his paper on " Leptodora " ("Zeitschrift f. 

 Wiss. Zoologie," Vol xxvii., 1876), and he also brought for- 

 ward a number of interesting details on the subject at the end 

 of his paper " Ueber die Schmuckfarben der Daphnoiden " 

 ("Zeit. f. Wiss. Zool.," Supplement, Vol. xxx, 1878). 

 Several other authors have from time to time added isolated 

 details to the facts already known, but these scarcely call for 

 detailed reference. 



Considering first of all the essential structure of the olfactory 

 setae of the Cladocera, it will be found that each seta consists 

 of an extremely hyaline and perfectly smooth rod, either 

 cylindrical or somewhat tapering towards its tip (see Figs. 

 2-6). The walls of the rod seem to be but very slightly 

 chitinised and very thin, whilst the internal protoplasm is 

 either homogeneous or, as is more usual, contains a number 

 of vacuoles and granules. The tip of the seta is always fur- 

 nished with a small highly refracting pellet, presumably of 

 chitin, and it may be useful to note that it is this pellet which 

 forms the most easily recognised mark of a cladoceran olfactory 

 seta. At its point of insertion on the antennule the hyaline 

 rod joins a strongly chitinised more or less elongated bead of 

 variable shape (Figs. 2-6), which is completely imbedded in the 

 antennule. This elongated bead is, at its innermost extremit}^, 

 joined by a delicate nerve-thread, or rather, perhaps, series of 

 nerve-threads, which can be traced back to the large ganglion 

 of nerve-cells forming such a prominent feature of the anten- 

 nule, and from the ganglion again it is easy to trace back the 

 nerve coming from the brain. 



To the foregoing facts of the essential structure, which were 

 practically all worked out by Ley dig, and have not since been 

 materially augmented, I am now inclined to add, on the 

 strength of a large number of observations, that the hyaline 



Journ. Q. M. C, Series II., No. 39. 20 



