46 KBPORT — 1855. 



Olfactory organ. — We have elsewhere* given our reasons for following 

 the opinion of Dr. Farre, in transferring the seat of this sense to the lower 

 or external antenna, in opposition to the opinions of Prof. Milne-Edwards, 

 Von Siebold, and others. These, since they are too recent to be generally 

 known, we shall here briefly recapitulate. 



" The question whicli we have to consider is, to which sense either of the 

 two sets of organs belongs ; — whether the upper belongs to the auditory and 

 the lower to the olfactory, as we shall endeavour to prove ; or vice versa, as 

 maintained by all previous writers, except Dr. Farre and Mr. Huxley. 



" We shall divide the evidences on either side under two heads ; first, that 

 which is derived from an external observation ; and second, that which is 

 derived from the internal organization. 



" First then from external circumstances : An auditory apparatus is an 

 organ furnished to an animal for one or both of two objects; first, for pro- 

 tection from danger; second, for the pleasure derivable from sounds. To 

 animals so low in the scale of being as the Crustacea, placed as they are 

 in a medium which must considerably modify its character, sound can convey 

 little to the consciousness of the animal beyond a sense of security or danger. 



" To enable this to be of the most extensive value, the auditory organ must 

 be, and always is placed so as to be most exposed to external impressions at 

 all periods ; particularly when the animal is at rest or pre-occupied. 



" Now if we look at the organ which the present state of science attributes to 

 the sense of hearing, we find that in the most perfectly formed animals, the 

 Brachyura, it is enclosed within a bony case and secured by a calcareous 

 operculum ; that it is always so in a state of rest, and only exposed when 

 especially required. Not only is this the case throughout the order, but in 

 some genera, as in Corystes, Cancer, &c., it is again covered by the supplying 

 organs of the mouth. 



" If we take into consideration the nature of sound, and its difference of 

 character when conveyed under water from that of passing through air, the 

 obtuse character of the former, which can scarcely be more than a vibratory 

 action of particles of water, which conveys to us a very modified and imper- 

 fect idea of sound, we find it difficult to understand that the organ situated 

 at the base of the under (internal) antenna is capable of receiving impressions 

 of sound, enclosed as it is within and covered by a stout calcareous oper- 

 culum. 



" But if we view it as an organ of smell, every objection previously mani- 

 fest now becomes evidence in i'avour of the idea. The small door, when it 

 is raised, exposes the orifice in a direction pointing to the mouth ; this also is 

 the direction of the same organ in all the higher orders. In Amphipoda it 

 is directed inwards and forwards. In every animal it is so situated, that it is 

 impossible for any food to be conveyed into the mouth without passing under 

 the test of this organ, and by it the animal has the power to judge the 

 suitability of the substance as food, by raising the operculum at will, and 

 exposing to it the hidden organ — the olfactory." 



The deductions in the paper just quoted were the result of researches 

 chiefly made on the Brachyura. In the Amphipoda, the homologue of the 

 above organ, which we maintain is adapted for smelling, is to be found in the 

 form of a small spine or denticle at the inferior side of the second antenna. 



This denticle is so constant, that its absence is a thing of note, as for 

 instance in the almost terrestrial genus of Orchestia; probably the result 

 of an adaptation of the internal organ to meet a more rarefied atmosphere. 



This organ appears to be developed from the first and second joints of the 

 * Annals of Natural History, July 1855. 



