ON THE BRITISH EDBIOPHTHALMA. 4Sk 



grtssive development. They are not the evidence of a more perfect 

 structure. 



This fact has not its full weight in the reasoning of Mr. Dana, when he 

 makes the short upper antennae evidence of a higher organized Crustacea. 



The antenna is reduced in length to fulfil certain conditions : in TalitruSy 

 because it is needless as an aquatic organ ; in Lysianassa and its near allies, 

 possibly as a more perfect one ; in the Hyperidce, with scarce an excep- 

 tion, on account of the impoverished character of the whole animal. 



Talitrus and Hyperia are generally admitted by naturalists to rank at the 

 opposite extremities of the order, and if generalization were to be adopted 

 from a too narrow observation, then at whichever extremity of the order it 

 was confined, the faulty conclusion would be enunciated which identifies a 

 short anterior antenna as typical of an improved organization, and, on the 

 other hand, one of a more feeble type. 



The most perfectly formed anterior antenna belonging to the Amphipoda 

 has always appeared to us to be that furnished with the most perfect and 

 largest number of those appendages which we have in this paper denomi- 

 nated as auditory cilia, since they enable the organ more completely to fulfil 

 its office. These membranous cilia we believe to be the external agents 

 by which a sensation analogous to sound is conveyed to the consciousness of 

 the animal. The imperfect nature of the organ is in accordance with our idea 

 of the imperfect condition of the sensation conveyed to an animal so low in 

 the scale of creation, conducted as it is by means of a medium so dense as 

 water. We have never been able to observe any traces of an internal organ 

 in this antenna, but in one or two species we have thought we detected 

 a nerve traversing the lower side to the extremity of the peduncle in 

 jEgina longispina and Amphitoe rubricata. This nerve terminates at the 

 roots of the first auditory cilia, which are placed at the extremity of the 

 peduncle, and are repeated throughout the length of the filamentary con- 

 tinuation, which appears to us to be a more or less extended base for the 

 support of these delicate organisms. The number of auditory cilia belong- 

 ing to the antenna bears no relative proportion to its length. They crowd 

 together where the limb is short, as in Plate XIII. fig. 1. Upon the more 

 lengthened member they generally are to be found, one at the further ex- 

 ti'emity of each small articulation. 



These auditory cilia are to be found only on the principal filament in all 

 the malacostracous divisions of Crustacea ; the complementary appendage, 

 however important, is never furnished with them. Their forms vary in 

 different species, but not to any very considerable extent ; occasionally they 

 will be found, as fig. c in PlateXIII., to terminate %vith a little tooth-like point; 

 very commonly they are seen with a kind of semiarticulation near the centre, 

 as in Tetromatus ; often they are quite simple, as in Lysianassa. But 

 the most typical form appears to be blunt at the extremity, equal in 

 breadth from the top to the bottom, with a sudden decrease near the centre, 

 that gives it an articulated appearance. They are compressed longitudinally, 

 instead of being round like hairs generally, and are extremely delicate in 

 structure, quite transparent, and almost invisible when compared with the 

 true hair^. They are membranous and flexible, and we should presume pecu- 

 liarly appropriated for the reception of impressions of a vibratile character. 



The concentration of theio organisms upon a short antenna, together with 

 tlw evident increase of diameter at the base of the peduncle, may be iudica- 

 tioiBs of an organ better adapted for the reception of sounds; but we have 

 not been enabled to distinguish that there is consequently any relative in- 

 crease of perfection in the organization of the entire animal. 



