ON OUR PRESENT KNOWLEDGE OF TOE CRUSTACEA. 41 



Crustacea, which contains no otolithe, but is a large chamber filled with a 

 fluid in which these hairs stand in great numbers. 



The third kind exists on the peduncle of the first antenna and on the 

 second joint of the second antenna, and, in Palccmon, on the uropoda ; in 

 Mi/sis they exist in the same member ; so that the function of hearing must 

 be considered as established in this part of the animal. In Palcemon squilla 

 the auditory hairs are replaced, as the animal increases in age, by those 

 of the ordinary kind. Dr. Henseu classifies the several forms of auditory 

 apparatus under separate heads : — 



1. Those which have one otolithe within the auditory chamber — as Lucifer, 

 Sergcstes, Mysis, Hippolytc, and Mastigopus. 



2. Those that have no otolithe and no auditory chamber— as Thysanopoda 

 and Pandalus. Dr. Hensen also mentions Al'nna, Erichthus, and Phyllosoma ; 

 but these being the immature forms of known Crustacea must be excluded 

 from his list. 



3. Those that have a chamber with numerous otolithes — as Palcemon, 

 Pasiphae, Crangon, Alphceus, Astacus, Qebia, Pagnrus, Palinurus, Ncphrops, 

 and Lithodes. 



4. Those that have a closed auditory chamber but no otolithe — as Por- 

 cellana?, Hippa, Pinnotheres, Myctiris, Ocypoda, Grapsus,Licpea, Sesarma, 

 Nautilograpsus, Platy card nits, Pilumnus, Chlorodius, Grtashnvs, Trapezia, 

 Carcinus, and Hyas. 



From the experiments which he made, Dr. Hensen found that the animals 

 living in water took no notice of sound made in the air, and that they were 

 only slightly affected by sounds made with a fife or bell in contact with a 

 membrane connecting the same with the water, the only effect being that the 

 crab would first jump and then quit the place. He has observed freshly- 

 caught specimens of Palcvmon ant en narius on the first experiment leap out 

 of the water when a sound was made against the side of the vessel. He per- 

 foimed various other experiments on distinct species, among others that of 

 removing the auditory apparatus from the tail of Mysis, and was disappointed 

 to find the powers of hearing were not interfered with as much as he had 

 anticipated. 



In experiments made with musical notes, he was induced to believe that 

 certain hairs vibrated to certain sounds. Under these conditions, Dr. Hensen 

 found that a certain hair, which only vibrated under one note, will, under a 

 different one, shake to the very base so powerfully that it cannot be 

 distinctly observed, and that as soon as the sound ceases the movement also 

 ceases. To illustrate the extent to which Dr. Hensen believes this to be 

 capable of being carried, he has drawn up a scale of musical notes adapted to 

 the various hairs which he thinks belong to this sense. 



As we descend in the scale of Crustacean forms the antenna; naturally 

 become simplified ; but as they lose their internal structural character they 

 increase their external functional arrangement. Thus in Amphipoda the 

 auditory chamber and otolithes are wanting, but in all the aquatic normal 

 forms the filaments are long, and richly studded with those membranous 

 organisms that I have named auditory cilia. 



The second pair of antenna? has a tendency to vary in form to a greater 

 degree than the first, but the functional variation is as limited. 



In the higher forms, such as the Brachyura, some of the joints of which 

 they consist are fused together, and not unfrcquently ossified with the 

 tegumentary tissues of the head or cephalon, in some instances to such an 

 extent that' their separation cannot be identified. But whether free or fused 



