OLFACTORY ORGAN 199 



two portions, an anterior and a posterior (Figs. 157, B, and 158), by 

 a fold of skin. The nostril often lies at the summit of a longer or 

 shorter tube, lined with ciliated cells, and the distance between 

 the anterior and the posterior aperture varies greatly, according to 

 the width of the fold of skin which separates them. 



The mucous membrane of the nasal organ of Fishes is always 

 raised up into a more or less complicated system of folds, which 

 may have a transverse, radial, rosette-like, or longitudinal arrange- 



E80 



Fig. 158. — Lateral View of the Head of Murima helena. 



VR and HB, anterior and posterior tubes of the external nostrils ; A , eye ; 

 HSO, integumentary sense-organs. 



ment, and which are supplied by the branches of the olfactory 

 nerve. 



The olfactory organ of Polypterus is more highly developed and compli- 

 cated than that of any other Fish. In certam representatives of the Pleoto- 

 gnathi and Gymnodontes amongst the Teleostei, on the other hand, the organ 

 shows various stages of degeneration. 



Dipnoi. — A nasal skeleton well differentiated from the skull 

 proper is met with for the first time in Dipnoans. In Protopterus 

 it consists of a cartilaginous trellis-work enclosing each olfactory 

 sac and united with its fellow in the median line by a solid septum : 

 the floor is formed mainly by the pterygopalatine and by con- 

 nective tissue. The mucous membrane is raised into numerous 

 transverse folds connected with a longitudinal fold, and the olfac- 

 tory organ in general most nearly resembles that of Elasmobranchs, 



