Chap. XII.] 



OLFACTORY ORGANS. 



439 



which can be definitely asserted to have an olfactory 

 function. In the higher Crustacea we find organs in 

 the antennules which, in the crayfish, are thus dis- 

 posed ; the outer branch (exopodite) has attached to 

 the greater number of its more distal joints tufts of 

 short delicate bristles, flattened or papilliform at their 

 free ends ; these bristles have granular contents, and 

 are supplied by fine nerve fibres. In the Insecta, 

 where there is only one pair of antennae, the olfactory 

 organ is, to judge from the 

 accounts of Braxton Hicks 

 and Lowne, placed in the 

 third joint of the antenna 

 of the blowfly ; the surface 

 of this joint is described as 

 being " covered with minute 

 hairs, between which are a 

 vast number of pellucid dots, 

 about 17,000 or 18,000 on 

 each antenna, with about 



'i,i . , 



eighty large irregular spots 

 of a similar character." The 

 smaller dots appear to be the optical expression of 

 the orifices of minute sacculi, and the larger the 

 common openings of compound sacculi. This third 

 antennal joint is described as being filled with a cellular 

 pulp, through which are distributed the fibrils of the 

 antennary nerve. 



In the Mollusca the olfactory organ ("os- 

 phradium," Lankester) is remarkable for its 

 constant relation to the neighbourhood of the respi- 

 ratory orifice, and its as constant nerve supply from 

 the visceral commissures \ it appears to be absent in 

 air-breathing forms (e.g. the snail), and we may 

 suppose, therefore, that it has a function in the way of 

 testing the water which carries the oxygen necessary 

 for respiration. It has ordinarily the form of a short 



i g- 187 A. Olfactory Appen- 



dasre of Exopodite of anteimule 



of Crayfish fx 800. a. Front, b. 



Side View - < After 



