VERTEBRATES. 39 



ends of the neuro-eijithelial cells, wliicli at first look into the cavity of the optic vesicle, 

 but, when that disappears, become surrounded by branched outgrowths of the pig- 

 mentary epithelial cells, which furnish the rods and cones with the sensitive visual pur- 

 ple. The relative proportion of rods and cones is very different in different groups, 

 and their structure is likewise different — those of the reptiles and birds being partic- 

 ularly remarkable for the brilliantly colored drops of oil which are present in them. 



The Olpactoey Organ. 



Like the auditory organ, the olfactory organ first appears as an involution of the 

 skin. It is represented by a ciliated sac in Amphioxus, the position of which is such 

 as to suggest that the organ was related to the anterior end of the open neural canal 

 in the earliest vertebrates, a theory which would explain the source of its nervous sup- 

 ])ly by the extreme anterior end of the prosencephalon. If correct, this theory would 

 lead us to believe that the unpaired sac met with in Amphioxus and the Myzonts is the 

 primitive condition, and that the paired condition in higher vertebrates is secondary. 

 A very remarkable difference exists between the nasal organ of the hag-fish and the 

 lamprey, which merits attention. In the latter fish the olfactory mucous membrane 

 is lodged in an unpaired sac, opening on the upper surface of the head, by means 

 of the remnant of the hypophysial duct referred to above, and has no communication 

 at all with the cavity of the mouth, while in the former, the hypophysial duct forms a 

 complete tube leading from the upper surface to the cavity of the mouth, and the ol- 

 factory mucous membrane is lodged in a posterior recess of this tube. The connec- 

 tion between the nasal and mouth cavities, which obtains in the higher vertebrates, is 

 brought about in a very different fashion. 



In fishes generally, the olfactory sacs are paired, and lined with mucous membrane 

 disposed in folds, in the grooves between which the special sensory cells are lodged, 

 while the surfaces of the folds are merely clad by ciliated cells which cause a current 

 to stream over them. The roofs of the sacs are frequently strengthened by bone, and 

 there are generally two apertures, often quite close to each other, which may be pro- 

 longed into tubes, through which the current makes its entrance and exit. 



In certain foi-ms the folds are so disposed as to have created the impression that 

 the olfactory sacs might represent a pair of modified gill-clefts with contained fila- 

 ments, but the theory of their origin, referred to above, is that which is more gener- 

 ally adhered to. 



In sharks a groove leads from the olfactory organ to the mouth, and no doubt this 

 is the first representative of the similar groove which connects on either side the 

 olfactory pit, and the cavity of the mouth in higher forms, and which afterwards is 

 closed into a canal leading from the olfactory sac into the mouth cavity. The pos- 

 terior apertures of these canals form the posterior nostrils or choanje ; they are situated 

 directly behind the upper lip in Amphibia, but, by the development of the shelf of the 

 palate referred to above (p. 18), gradually shift further backward until they occupy a 

 position far back in the cavity of the mouth. 



Mention has already been made of the complex turbinal surface afforded for the 

 olfactory mucous membrane by the nasal capsule. Only part of the cavity is occupied 

 by the olfactory neuro-epitheliuin, the rest serving as a channel for respiration in the 

 higher forms, where the posterior nostrils are brought into close relation with the top 

 of the windpipe. The whole mucous surface is provided with glands, which furnish 

 the moisture necessary to preserve the functional activity of the epithelium. A re- 



