532 



OLFACTORY PIT. 



would not be profitable to deal with them more fully in this 

 place. Amongst the Chordata there are usually well developed 

 olfactory organs. 



Amongst the Urochorda (Tunicata) it is still uncertain what 

 organs (if any) deserve this appellation. The organ on the 

 dorsal side of the opening of the respiratory pharynx may very 

 possibly have an olfactory function, but it is certainly not homo- 

 logous with the olfactory pits of the true Vertebrata, and as 

 mentioned above (pp. 436 and 437), may perhaps be homologous 

 with the pituitary body. 



In the Cephalochorda (Amphioxus) there is a shallow ciliated 

 pit, discovered by Kolliker, which is situated on the left side of 

 the head, and is closely connected with a special process of the 



au.n 



hit 



FlG. 307. VIEWS OF THE HEAD OF ELASMOBRANCH EMBRYOS AT TWO STAGES 



AS TRANSPARENT OBJECTS. 



A. Pristiurus embryo of the same stage as fig. 28 F. 



B. Somewhat older Scyllium embryo. 



///.third nerve; V. fifth nerve; VII. seventh nerve; an.n. auditory nerve; gl. 

 glossopharyngeal nerve; Vg. vagus nerve ; fb. fore-brain; pn. pineal gland; mb. mid- 

 brain ; hb. hind-brain ; iv.v. fourth ventricle ; cl>. cerebellum ; ol. olfactory pit ; op. 

 eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; ht. heart; 

 Vc. visceral clefts; eg. external gills; pp. sections of body cavity in the head. 



