THE OLFACTORY ORGANS. 115 



nature, developed on the under side of the outer branch 

 of the antennule, play the part of an olfactory apparatus. 

 Both the outer (fig. 26 A. ex) and the inner (en) 

 branches of the antennule are made up of a number of 

 delicate ring-like segments, which bear fine setae (b) of 

 the ordinary character. 



The inner branch, which is the shorter of the two, pos- 

 sesses only these setse ; but the under surface of each of 

 the joints of the outer branch, from about the seventh or 

 eighth to the last but one, is provided with two bundles 

 of very curious appendages (fig. 27, A, B, C, a), one in 

 front and one behind. These are rather more than 

 l-200th of an inch long, very delicate, and shaped like a 

 spatula, with a rounded handle and a flattened somewhat 

 curved blade, the end of which is sometimes truncated, 

 sometimes has the form of a prominent papilla. There 

 is a sort of joint between the handle and the blade, such 

 as is found between the basal and the terminal parts of 

 the ordinary setae, with which, in fact, these processes 

 entirely correspond in their essential structure. A soft 

 granular tissue fills the interior of each of these pro- 

 blematical structures, to which Leydig, their discoverer, 

 ascribes an olfactory function. 



It is probable that the crayfish possesses something 

 analogous to taste, and a very likely seat for the organ 

 of this function is in the upper lip and the metastoma ; 

 but if the organ exists it possesses no structural pecu- 

 liarities by which it can be identified. 



