AN ACOUSTIC TAIL 37 



appreciable, and opens into a calcined 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 

 Mysidse which, extraordinary to relate, carry it in the 

 tail (see Plate XIII.). In some of the Amphipoda otoliths 

 have been detected in connection with the brain, not in, but 

 behind the antennae. In general, the antennas 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 Mysidse are connected with the caudal 

 otoliths. The principal flagellum of the upper antennas 

 is frequently furnished with a number of smooth setae or 

 filaments, which were at one time described as auditory 



7 



cilia, though there was nothing to support this guess at 

 their function, and though the term cilium was inappro- 

 priate to the shape of these rod-like membranous filaments. 

 It was noticed by various naturalists of eminence that the 

 setae of this form were much more abundant in the adult 

 males than in the young males or in the females. Ley dig 

 supposed them to be not auditory but olfactory organs, 

 and Fritz Muller independently 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 Nanplii 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. 



