OLFACTORY ORGANS 



207 



developed for drawing air over the sensory surface, the first traces of 

 which are seen in the elasmobranchs. These frequently have an 



Fig. 215, Fig. 216. 



Fig. 215. — Section through one nasal labyrinth of Polypterus. The nerve runs 

 through the centre. 



Fig. 216. — Head of skate {Mlobalus) after Jordan-Evermann, showing oronasal 

 groove {0) leading from nans (n) to mouth. 



oronasal groove, leading from each naris to the angle of the mouth 

 (fig. 216). In some species this groove is practically converted into 



■/ 



Flc. 217. 1 II.. ;iS. 



Fig. 217. — Head of chick of 5H da^, showing development of oronasal canal, 

 after Keibel. cf, chorioid fissure; /, thickening for lacrimal duct; n, nasal pit; on, 

 oronasal groove. 



Fig. 218. — Model of mouth of Echidna embryo, after Seydel, showing method of 

 ingrowth of palatal folds (^/) to cut off secondary nasal passages, ch, primitive choanae; 

 et, egg tooth;/, opening of Jacobson's organ. 



a tube^by the meeting of the walls below. Beginning with the dipnoi 

 and continuing with the amphibia and amniotes (fig. 217) a similar 

 groove is formed on either side before the formation of skeletal parts 



