IV. VERTEBRATA. 475 



The olfactory epithelium of many rishcs and amphibia is a stratified 

 epithelium with closely arranged end buds (fig. 529;. It is conceivable 

 that by disappearance of the isolating parts of ordinary epithelium the end 

 buds would form a continuous sensory epithelium, which is the rule in 

 most vertebrates, yet this view meets difficulties in the finer structure of 

 the olfactory cells. 



The olfactory organ, the nose, lined with its sensory epithelium, ac- 

 quires a special interest both from its grade of development and from the 

 important systematic distinctions it affords. Except the cyclostomes, 

 which have an unpaired nasal sac, all vertebrates have paired olfactory 

 organs. In adult fishes and in the embryos of higher forms there are two 

 pits which lie in front of or dorsal to the mouth; they are either distinct 

 from it or only connected with it by an oronasal groove in the skin (fig. 

 556). If the animal be terrestrial and replace branchial by pulmonary 

 respiration, a respiratory canal is developed in connection with the nose. 

 The oronasal groove closes to a tube which begins with a nostril (nans) on 

 the surface and ends with a second opening (choana) in the mouth cavity. 

 The olfactory sac proper is included in the wall of this tube, usually on its 

 dorsal surface (fig. 530). In Amphibia, lizards, snakes, and birds the 

 choana is far forward, behind the upper jaw; in alligators, turtles, and 

 mammals it is far back, in crocodiles and some mammals (edentates) 

 nearly to the vertebral column. This is brought about by the develop- 

 ment of the hard palate, a parting wall which divides the primitive mouth 

 cavity into two portions, a lower, the persistent or secondary mouth 

 cavity, and an upper, which, as a secondary nasal cavity, continues the 

 passage backwards. The bones of the maxillary and palatine series con- 

 tribute to the hard palate, sending out horizontal processes which meet 

 in the middle line. In mammals and crocodiles this partition is con- 

 tinued back by the soft palate. 



A further increase in the nasal cavity is brought about by complicated folds 

 in the walls supported by special skeletal parts, the tnrbinal bones, and also by 

 the outgrowth of chambers, lined with mucous membrane which extend into the 

 neighboring bones. Again, a part of the primitive chamber lined with olfactory 

 epithelium can be cut off from the rest and form an accessory nose, Jacobson's 

 organ (fig. 530, P), which opens into the mouth behind the premaxillaries by 

 Stensori's duct. This organ is best developed in lizards, monotremcs and un- 

 gulates, but often occurs in a reduced condition in other terrestrial vertebrates. 



In all vertebrates, except Myxlne and a few forms living in the dark, 

 the eyes are composed of all the principal constituents which occur in the 

 human eye and which have already been briefly described (p. 1 19, fig. 86). 

 In most vertebrates it is nearly spherical, the optic nerve entering it from 

 behind, its interior occupied by transparent, refractive substances (lens. 



