40 KEPORT — 1877. 



genera in all the Podophthalmous Crustacea, whether Brachyura, Macrura, or 

 Stomapoda, there are universally present two or more of these filamentary 

 appendages, often subequally long, only one of which, the primary, appears 

 to fulfil any important office. 



In Amphipoda there is never more than one secondary appendage, and 

 that is always of a rudimentary character, and frequently only determinable 

 iu the very young stage of the animal and obsolete in the adult. In the 

 Isopoda, with the exception of the Anisopod group, it is always absent. 



The secondary appendage, even in those families where it is most de- 

 veloped, appears to fulfil but an unimportant office. 



In this it differs from the principal filament, or tige, as it is named by 

 Milne-Edwards, which, in addition to the numerous simply-fornicd hairs 

 with which it is covered, is furnished with a considerable number of mem- 

 branous cilia, which arc peculiar to this organ in Crustacea, and may be 

 found in every form of animal iu the class, except where the entire appendage 

 has become impoverished from the peculiar nature of the animal's habits 

 or conditions, such as in the Terrestrial Isopods or parasitic families of 

 Crustacea. 



The forms of these cilia vary a little in separate genera ; but in whatever 

 Bhape they are found there is, I think, no doubt but that they arc actively 

 concerned in communicating vibrations, analogous to the waves of sound, to 

 the nerve-system in this pair of antennae ; and on this account it is that 

 I named them, in my lieport on the Sessile-eyed Crustacea to this Asso- 

 ciation in 1855, as being auditory cilia. 



In 1853 M. Lcuckart (Troschel's ' Archiv,' i. p. 255) stated that the 

 organ attributed to auditory consciousness was not to be found at the base of 

 the antenna) in the genus M>/$is as in other Crustacea, but that a chamber 

 containing an otolithe, similar to that found in the antennae of Macrura, 

 existed in the inner ramus of the caudal plcopoda, which has been confirmed 

 by Ivrbyer, Van Beneden, and, I can add, my own observations. Kroyer and 

 Van Beneden have traced the branch of a nerve to the chamber, and have 

 no doubt but that this organ fulfils the functions of an auditory apparatus. 



This small otolithe, according to Van Bcncden's description, shows an 

 extreme regularity in the arrangement of the several layers of which it is 

 formed, as if it were a little agate or highly polished siliceous stone. 



It is liable to vary somewhat in form in separate species. 



In 1863 Dr. V. flensen published his researches on the auditory organs 

 of Decapod Crustacea (Zeitsehr. f. wissensch. Zool. xiii. Bd. 3 Hit. 1863). 



His observations extended to twenty- eight distinct forms, and he confirmed 

 the assertions of Farre and others that the sand found in the auditory chamber 

 of the Prawn &c, is but common sand, ho having seen the animal introduco 

 it after having ruovdted. But in those Crustacea in which the auditory 

 chamber is closed and the otolithes are cast at every moult and again repro- 

 duced, these organisms Professor Huniby, after having tested some 200, thinks 

 to be fluate of lime. 



Dr. Hensen, however, appears to attribute more of the power of hearing 

 to the hairs of certain forms that exist, (1) attached to the otolithes; 

 (2) attached to the auditory chamber ; (3) attached to the external surface of 

 the animal. 



The first of these he finds to exist chiefly among the Macrurous Decapoda, 

 in some cases springing from among the otolithes, in others supporting, as in 

 the tail of Mysis, the otolithe in its position. 



The second kind of hair exists in the auditory chamber of the Brachyurous 



