102 



ZOOLOGY 



SECT. 



the exterior by an aperture, the external nostril. It is paired in 

 all Craniata, except Cyclostomes, in which there is a single olfactory 

 sac, supplied, however, by paired olfactory nerves. The sac is lined 

 by the olfactory mucous membrane or Schneidericm membrane, 

 the epithelium of which contains peculiar, elongated sensory cells 

 (Fig. 738), their free ends often produced into hair-like processes. 

 In the Dipnoi and all higher groups the posterior end of each sac 



/ R 



A 



FIG. 736. A, diagram of the organs of the lateral line in a Fish. .;-, lateral line ; ad, its 

 continuation on the head. B, organ of the lateral line in a tailed Amphibian (semi- 

 diagrammatic), a, epidermic cells, through which are seen b, sensory cells ; <-, sensory hairs ; 

 N, nerve ; R, hyaline tube. (From Wiedersheim's Vertebrate,.) 



communicates with the cavity of the mouth by an aperture called 

 the posterior nostril, and a similar communication occurs in the 

 case of the unpaired organ of the Hags. 



In many air-breathing Vertebrates there is formed an offshoot 

 from the olfactory organ, which, becoming separated, forms a 

 distinct sac lined with olfactory epithelium and opening into the 

 mouth. This is Jacobsons Organ : it is supplied by the olfactory 

 and trigeminal nerves. 



The paired eye is a more or less globular structure, lying in 



