OLFACTORY SET J) OF THE CLADOCERA. 283 



is attached, and which lies wholly within the antennule. The 

 fundamental form of this seems to be a hollow cylinder. In 

 most, if not all cases, however, some departure from this simple 

 form is shown. Occasionally the proximal cud is enlarged 

 whilst the remaining portion retains the cylindrical shape, or 

 nearly so, as in Leptodora. Much more frequently it is the 

 distal end which is enlarged, and perhaps still more often 

 both ends are enlarged, producing the appearance of a dumb- 

 bell. From these three main types a very great number of 

 different and characteristic shapes are produced, owing to the 

 various degrees of enlargement of the ends and the length of 

 the whole structure. 



Comparing the olfactory setae as above described with those 

 from other groups of the Crustacea (the typical Crustacean olfac- 

 tory seta is well shown by Asellus aquaticus, Fig. 7), the most 

 noticeable difference is the absence of the strongly chitinised 

 stalk or handle in the former. This absence is, I believe, more 

 apparent than real, however, for it is more reasonable to sup- 

 pose that this part of the seta is really represented by the 

 elongated basal bead than that the latter is an entirely new 

 structure. It is true Leydig was of the opinion that the highly 

 chitinised beads within the antennule were really depressions 

 in the epidermis, and it is very probable that they were actually 

 formed in this way in the first instance, although they do not 

 show much trace of such a structure now. However this may 

 be, it is still highly probable that the beads in the cladoceran 

 antennule represent the stalks of the olfactory setae in most other 

 forms of Crustacea, for the setae would necessarily arise from the 

 bottom of the pits when such existed. 



Taking next the question of the number of olfactory setae in 

 each tuft it must be noted that hitherto no definite rule appears 

 to have been observed, although in numerous descriptions of 

 species the number of setae has been alluded to. Statements 

 may be found in papers by Kurz, Hellich, Weismann, and 

 others, that from 5 to 10, 6 to 7, 6 to 8, 7 to 9, 8 to 10 such 

 setae are present in various cases. Even in one of the most 

 recent works on the Cladocera (" Revision des Cladoceres," by 

 J. Richard, " Annales des Sciences Naturelles, Zoologie," "Vol. 

 xviii., 1895) it is said that the anterior antenna carries " un 

 nombre de soies speciales qui oscille autour de 6. II y en a 



