44 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



canal, which either ends blindly, or is bifurcated at its 

 free end ; at this end, or at the point of bifurcation, 

 there is a small ganglion. The cylindrical canal 

 consists of a network of coiled fibrous bands, and is 

 invested by elongated epithelial cells, which are 

 directly continuous with the integument ; these cells 

 are very richly supplied with nerve fibres. 



Among the Chordata no definite olfactory organ 

 has been recognised in the Urocliordata ; in the 

 rest it always stands in close relation to the respiratory 

 orifice, but in nearly all fishes it is not directly 

 continuous with the respiratory passages. The single 

 pit at the anterior end of the body of Amphioxus, 

 though lined with a ciliated epithelium, can by no 

 means be certainly said to be an olfactory organ. 

 The Cyclostomata have but a single pit, whence they 

 have been distinguished from all other Vertebrata as 

 the Monorrhiiia ; notwithstanding the single con- 



O o 



dition of this pit the nerve supply is double, and we 

 must not, therefore, yield to the temptation to regard 

 this condition as being a primitive one ; in this, as in 

 many other points, the existing Cyclostomata show 

 that they stand at some distance from the primitive 

 vertebrate stock ; their single nasal pit is, almost 

 certainly, the result of the fusion of two originally 

 separate sacs ; this view is supported by the observation 

 that, in the larval lamprey, the sac is more nearly 

 divided into two internally than it is in the adult. 

 The interior of the cavity is occupied by folds, some 

 of which project farther inwards than others, and all 

 of which are covered by a mucous membrane ; to this 

 are distributed branches of the olfactory nerves. In 

 the lampreys the sac is closed posteriorly, but in 

 Myxinoids it opens into the cavity of the mouth. 



In all the rest of the Yertebrata the olfactory 

 organs arise from a pair of patches of epiblast in front 

 of the mouth, which, as they thicken, give rise to a 



