AN ACOUSTIC TAIL 37 
appreciable, and opens into a calcified chamber, more or 
less filled with particles of sand, which are voluntarily 
placed in position by the animal soon after casting its 
exuvium. But while the higher Podophthalma have the 
organ of hearing thus placed, there are some—the 
Mysidee—which, extraordinary to relate, carry it in the 
tail (see Plate XIII.). Insome of the Amphipoda otoliths 
have been detected in connection with the brain, not in, but 
behind the antenne. In general, the antenne are fur- 
nished with delicate plumose hairs, the vibrations of which 
assist in the conveyance of sound to the auditory nerves. 
Similar hairs in the Mysidze are connected with the caudal 
otoliths. The principal flagellum of the upper antennz 
is frequently furnished with a number of smooth setz or 
filaments, which were at one time described as auditory 
cilia, though there was nothing to support this guess at 
their function, and though the term czliwm was inappro- 
priate to the shape of these rod-like membranous filaments. 
It was noticed by various naturalists of eminence that the 
setze of this form were much more abundant in the adult 
males than in the young males or in the females. Leydig 
supposed them to be not auditory but olfactory organs, 
and Fritz Miiller inde~endently came to the same conclu- 
sion, adducing in support of it their stronger development 
in the males, as in other cases male animals are guided by 
the scent in pursuit of the females. It can scarcely be 
said that their olfactory function is as yet absolutely 
proved, but they are evidently not well placed to serve 
the sense of taste, and for the senses of sight, hearing, 
and touch, there are other organs much better adapted, 
so that these glassy filaments, to be sensory organs at all, 
are in a manner forced back upon the sense of smell. 
The secondary or inner flagellum, according to Dr. 
Boas, is wanting in all genuine Nauplii—that is, in the 
earliest larval stage of the crustacean. Its after develop- 
ment conforms to no known rule, since in some species it 
is not found at all, in others it is only rudimentary, 
whereas, on the other hand, among the Macrura it is not 
unfrequently much longer than the outer flagellum. Mr, 
