OLFACTORY ORGAN 263 



two apertures varies greatly, according to the width of the fold of 

 skin which separates them (Fig. 191, B and c). A passage for 

 the water is thus here also present, but, unlike that of Elasmo- 

 branchs, is not connected with the mouth. 



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- 

 ment, and on which the sensory cells, as well as ciliated cells, are 

 situated. 1 



A nasal skeleton which is 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 the 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, 

 except that, as already mentioned (p. 259), internal as well as 

 external nostrils are present. The latter open beneath the upper 

 lip, and so cannot be seen when the mouth is closed ; the former 

 <>lcn into the oral cavity rather further back. 2 



Amphibians. - - The olfactory organ of Perennibranchiates 

 resembles in many respects that of the Dipnoans : it is always 

 enclosed within a complete or perforated cartilaginous capsule 

 situated laterally to the snout close beneath the skin, and is not 

 protected by the bones of the skull (Fig. 192). Its floor is largely 

 fibrous, and the mucous membrane is raised into radial folds like 

 those of Cyclostomes and Polypterus. 



In the higher Amphibia the olfactory organ is more com- 

 pletely included within the cranial skeleton, and its structure 

 becomes modified in correspondence with the change in the 

 mode of respiration, the nasal chamber giving rise to a special 

 respiratory portion, into which the external and internal nostrils 

 open. In Urodeles, the lumen of the organ is from the first 

 simple, while in Anurans, dorsal, middle, and ventral portions 

 may early be distinguished ; but in both cases the cavity becomes 



1 The olfactory organ probably reaches its highest development and most 

 complicated form amongst Fishes in Polypterus. The nostril leads into an outer 

 cavity, which communicates with the olfactory sac proper, and the latter is 

 divided up into six radial compartments arranged around a central spindle and 

 separated by complicated septa, so that a transverse section of the organ some- 

 what resembles in appearance that of an orange. In certain representatives of 

 the Plectognathi and Gymnodontes amongst Teleosts, on the other hand, the organ 

 shows various stages of degeneration, and may even undergo almost entire 

 reduction. 



' The peculiar position of the anterior nares has a physiological significance, 

 at any rate in Protopterus, in connection witli the habits of the animal ; during its 

 summer sleep the animal breathes through a tube, passing between the lips, formed 

 from the capsule or cocoon which encloses it. The necessary moisture for the 

 olfactory mucous membrane during this time is provided by the numerous goblet 

 cells which line the walls of both nostrils (cf. p. 20). 



