■220 



COMPARATIVE ANATOMY 



In the Cetaoea, the lachrymal and Meibomian glands, as well as the naso- 

 lachrymal duct, are wanting, and alachrymalduct is absent in the Otter, Seal 

 and Hippopotamus. In the two last-mentioned animals the lachrymal gland 

 is much reduced : in Manis javanica there are no Meibomian glands, and in 

 the Mole the entire lachrymal apparatus has undergone reduction. 



Auditory Organ. 



It is very probable that the auditory organ, like the organs 

 of smell and taste, has been derived primitively from a modified 

 integumentary sense-organ. It is developed from an invagination 



Fig. 176. — Head and Antbriok 

 PoETioN or Body of a Chick. 

 (In part after Moldenhauer. ) 



RG, olfactory pit ; A, eye ; / to 

 IV, first to foiirth visceral 

 arches ; t, point at which the 

 external auditory passage begins 

 to be formed ; LB, primitive 

 auditory vesicle seen through 

 the wall of the head. 



Fig. 177. — Semidiaorajoiatic Figdee of 

 THE Membranous Labyeisth of Verte- 

 brates. (Seen from the outer side.) 



u, utriculus ; rec, recessus utriculi ; sp, sinus 

 posterior utriculi ; s, sacculus ; I, recessus 

 sacculi (lagena) ; cus, utriculo-saccular 

 canal ; de, se, ductus and sacous endolym- 

 phaticus, the former arising from the 

 sacculus at t ; ss, sinus utriculi superior ; 

 ass, apex oi the same ; ca, ce, cp, anterior, 

 external, and posterior semicircular canals; 

 aa, ae, aji, the corresponding ampullae. 



of the ectoderm on either side of the primary hind-brain : this be- 

 comes separated off to form a vesicle (Fig. 176), and its epithelium 

 is differentiated into elongated cells of sensory cpithelmm pro- 

 vided with hair-like processes (Figs. 178 A and B) separated by 

 su2}2}ortinff cells. The sensory cells are surrounded by a nerve- 

 network, and are not continuous with the nerves as in the case of 

 the olfactory cells (p. 197). 



Like the other higher sense-organs, the paired auditory organ 



