OLFACTORY ORGAN. 171 



These facts in the structure and development of the olfactory organ and 

 nerve have caused an attempt to be made to draw a parallel between the 

 olfactory pit and a primitive preoral gill-cleft, 1 and this is further supported 

 by the general structure and histological relations of the olfactory mucous 

 membrane, which corresponds with that of the gills of Fishes in the possession 

 of end-bulbs. In a recent paper, however, Beard has put forward the view 

 that "the nose is really a branchial (segmental) sense-organ, i.e. the sense- 

 organ of a non-existent gill-cleft, and not a gill-cleft itself." 



Fishes. In Petromyzon (Fig. 49, N, No) and Myxin- 

 oids the olfactory organ consists of a sac, unpaired externally, 

 lying close in front of the cranial cavity, and opening on the 

 dorsal surface of the anterior part of the head by a longer or 

 shorter chimney-like tube. Its mode of development and internal 

 structure, however, as well as the double olfactory nerve, seem to 

 prove that the olfactory organ of Cyclostomes must also have been 

 primitively a paired structure. 2 



The position of the olfactory organ in Elasmobranchs differs 

 from that of Cyclostomes in lying on the under instead of the 



I 



FIG. 141. ANTERIOR PORTION OF HEAD OF Acipcnser sturio. 



a, anterior, b, posterior opening of external nostrils ; o, isolated rosette of olfactory 



folds. 



upper surface of the snout. From these Fishes onwards throughout 

 all Vertebrates the organ is always paired, and is more or less 

 completely enclosed by a cartilaginous or bony investment, which 

 forms an outwork of the skull. 



From the Ganoids onwards it always has a similar position with 

 regard to the skull, being situated between the eye and the end of the 

 snout, either laterally or more or less dorsally. In the course of 

 development each external nostril of Ganoids and Teleostei becomes 

 divided into two portions, an anterior and a posterior (Fig. 141, a, 6, 

 and Fig. 142, AN, AX*}, by a fold of skin. The anterior often lies 

 at the summit of a longer or shorter tube, lined with ciliated cells, 

 and the distance between it and the posterior aperture varies 



1 According to this view, the condition which is seen in ilyiinoids and Dipnonns is 

 to be looked upon as the more primitive, and that of all other Fishes as secondary. 



8 It is improbable that the naso-palatine duct, which opens into the oral 

 cavity in Myxinoids, but ends blindly in Petromyzon, is directly comparable to the 

 posterior nares of higher Vertebrates. 



