CHAP. III.] OF SENSE ORGANS. 63 



the diffusion of odours, though it will not hold for the 

 diffusion of light. 



From what I have said, it may be inferred that, as re- 

 gards the delicacy of their respective physical causes, the 

 sense of Smell occupies a strictly intermediate position 

 between those of Taste and Sight. 



Although a rudimentary sense of Smell seems unques- 

 tionably to be possessed by such aquatic forms of the 

 invertebrata as Crustacea and the higher Mollusks, it is, 

 perhaps, a sense-endowment which generally exists to a 

 more developed and varied extent amongst air-breathing 

 animals. But in whatever forms of life it may be met 

 with, this sense-endowment seems to be always very 

 largely related to the detection and capture of food. In 

 this direction it comes to the aid of the already existing 

 senses of Sight, Touch, and Taste. It has, however, the 

 peculiarity of being scarcely otherwise called into 

 activity amongst invertebrate animals. 



Although we have so little positive knowledge con- 

 cerning the situations of Organs of Smell in invertebrates, 

 there is good reason for believing that they will (when 

 present) always exist in close proximity to the mouth. 

 It seems possible that in Crustacea they are to be found 

 at the base of the antennules ; that in Cephalopods they are 

 represented by two little fossae in the neighbourhood of 

 the eyes ; and that in Insects a power of appreciating 

 odours may be possessed either by the antennae them- 

 selves, or by a pair of fossae near their bases. Another 

 cephalic organ has also been referred to as possibly en- 

 dowed with a power of being impressed by odours. Owen 

 says :* " The application by the common house-fly of the 

 sheath of its proboscis to particles of solid or liquid food 

 before it imbibes them, is an action closely analogous to 

 * " Comp. Anat. of Invertebrate Animals," p. 368. 



