438 OLFACTORY PIT. 



Vertebrata. 



(386) A. Boettcher. "Bau u. Entwicklung d. Schuecke." DenkscliriJ'ti'ii d. 

 kaiscrl. Leap. Carol. Akad. d. Wissens lift ft., Vol. xxxv. 



(387) 0. Hasse. Die rergleich. Morphologic u. Histologie d. htlutigen Geliororgane 

 d. Wirbeltldere. Leipzig, 1873. 



(388) V. Hensen. "Zur Morphologic d. Schnecke." Zeit. f. wiss. Zool, Vol. 

 xin. 1863. 



(389) E. Huschke. "Ueb. d. erste Bildungsgeschichte d. Auges u. Ohres beim 

 bebriiteten Kiichlein." Isis von Oken, 1831, aud Meckel's Arcltiv, Vol. vi. 



(390) Keissner. De Auris internee formatione. Inaug.Diss. Dorpat, 1851. 



Accessory parts of Vertebrate Ear. 



(391) David Hunt. "A comparative sketch of the development of the ear and 

 eye in the Pig." Transactions of the International Otolof/ical Congress, 1876. 



(392) W. Moldeuhauer. ' " Zur Entwick. d. mittlereu u. ausseren Ohres." Mor- 

 phoL Jahrbuch, Vol. in. 1877. 



(393) V. Urbantschitsch. " Ueb. d. erste Anlage d. Mittelohres u. d. Trorn- 

 melfelles." Mittheil. a. d. embryol. Instit. Wicn, Heft i. 1877. 



Olfactory organ. 



Amongst the Invertebrata numerous sense organs have been 

 described under the title of olfactory organs. In aquatic animals they 

 often have the form of ciliated pits or grooves, while in the Insects and 

 Crustacea delicate hairs and other structures present on the antenna) 

 are usually believed to be organs of smell. Our knowledge of all these 

 organs is however so vague that it would not be profitable to deal 

 with them more fully in this place. Amongst the Ohordata 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 homologous with 

 the olfactory pits of the true Vertebrata, and as mentioned above 

 (pp. 359 and 3GO), may perhaps be homologous with the pituitary 

 body. 



In the Cephalochorda (Ainphioxus) 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 front end of the 

 brain. It is most probably the homologue of the olfactory pits of 

 the true Vertebrata. 



In the true Vertebrata the olfactory organ has usually the form of 

 a pair of pits, though in the Cyclostomata the organ is unpaired. 



In all the Vertebrata with two olfactory pits these organs are 

 formed from a pair of thickened patches of the epiblast, on the under 

 side of the fore-brain, immediately in front of the mouth (fig. 307, ol). 

 Each thickened patch of epiblast soon becomes involuted as a pit 

 (fig. 308, N), the lining cells of which become the olfactory or Schnei- 

 derian epithelium. The surface of this epithelium is usually much 

 increased by various foldings, which in the Elasmobranchii arise very 

 early, and are bilaterally symmetrical, diverging on each side like 

 the barbs of a feather from the median line. They subsequently 



