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 sete (b) of 
the ordinary character. 
The inner branch, which is the shorter of the two, pos- 
sesses only these sete; 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 
1-200th 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 ot 
the ordinary sete, with which, in fact, these processes 
entirely correspond in their essential structure. A so‘t 
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. 
12 
