&mell. 477 



\ on is there converted into a nervous current which passes along the 

 olfactory nerve fibres into the brain. It is essential that the body to be 

 smelt be soluble, so that it can be taken up by the mucous fluid of the 

 pituitary membrane. The essential action seems to be first the pro- 

 duction of chemical action in the cells of the pituitary membrane, which 

 then gives rise to the electro-nervous current. 



The sense of smell is a very ancient sense. In some of the T.urbel- 

 lariae, or Gliding-worms, there are olfactory organs consisting of two 

 mere indentations or pits above and behind the mouth, but not connected 

 with it. This arrangement must have in ancient times been persistent 

 through a long line of animal forms, now extinct, which formed the 

 connection between worms similar in construction to the present gliding- 

 worms, and the lower vetebrates of the equivalence of the selachian 

 fishes. The selachian fishes are by no means the lowest living verte- 

 brates, but the cyclostomi ( Hags and Lampreys ), the amphioxus and 

 the ascidian larvae, which constitute lower vertebrate forms, have but a 

 single olfactoiy organ in the middle, and so the}" would seem to belong, 

 not to the direct, but to a collateral line. ( The single nostrilled animals 

 are, however, bilateral as to eyes, ears and gill pouches.) 



In the stage of development following that of the selachian fishes, the 

 olfactory furrows become connected with the mouth, as the organs 

 appear permanently in some of the higher selachian fishes. There is, 

 as yet, no closed tube, but an open gutter, one end of which extends 

 to the corner of the mouth. Later, as the gutter sinks into the face, 

 the opposite edges reach over the channel towards each other, finally 

 coalescing and forming a tube, one end of which pierces the lip into the 

 mouth cavity, while the other is open forward. In this stage the organs 

 are like those permanent in the mud-fishes ( dipneusta ). l In most fishes 

 the nasal organs never become tubes, and are never connected with the 

 mouth. They are merely superficial sacs lined with the continuation of 

 the outer skin which is differentiated into sensitive mucous epithelium. 

 In the progressive evolution of this fish nostril, the membranous lining 

 becomes wrinkled into folds or ridges, which, in some cases, radiate from 

 a center, in others are arranged in parallel direction on each side of a 

 central band. In this way a greater surface of sensitive membrane is 

 exposed to the action of odorous particles, and those fishes possessing 

 the greatest complication of these folds have the most acute sense of 

 smell ( Agassiz). These folds call to mind the membrane covered tur- 

 binated bones of the human nose, in which the same end is accomplished 

 in :i very similar wa} T . In some fishes the first pair of nostrils are rein- 

 forced by a second pair, giving. them four in all instead of two. In the 

 further development of the embryonic nasal cavities, the ends of the 



1 The dinneusta are rather in advance of this stage, their nostrils opening somewhat 

 behind the lips. 



