396 COMPARATIVE ZOOLOGY OF CHORDATES 



in its wall. One of these is the fenestra ovalis which enables 

 the vibrations to be imparted to the fluid (perilymph) which 

 bathes the auditory sac. The other is the fenestra rotunda ; 

 it is covered by a membrane which absorbs the vibrations in 

 the perilymph and so brings them to an end. 



That part of the auditory sac which is actually concerned 

 with hearing is the cochlea, rudimentary in amphibia but well 

 developed in the higher vertebrates. The vibrations of the 

 perilymph are imparted to the endolymph within the cochlea, 

 which in its turn stimulates the sensory cells. In mammals 

 where the development of the ear is at its highest, the auditory 

 ossicles are three in number, the cochlea is long and coiled, 

 and an external ear assists in collecting the air vibrations. 



Cyclostomes, fish, and larval amphibia possess a system of 

 sense-organs known as the lateral-line organs, and which serve 

 to appreciate vibrations in water of low frequency. The ear 

 itself is to be regarded as a specialised organ of the lateral-line 

 (or " neuromast ") system. 



The Nose. — The olfactory organ or nose contains an 

 epithelium which is sensitive to very minute quantities of 

 chemical substances, dissolved or suspended in water, or 

 suspended in air. In Dipnoi and Tetrapods the nose has an 

 open connexion with the mouth cavity, and so enters into the 

 service of the respiratory system, enabling air to reach the 

 lungs without opening the mouth. This connexion does not 

 exist in forms below the Dipnoi (except in Myxine, where the 

 hypophysial sac opens into the gut). 



Taste-organs. — The nose is a distance-receptor, appreciat- 

 ing chemical substances from afar. Taste-organs, on the other 

 hand, serve for appreciating substances in contact with the 

 animal, and especially in connexion with the opening of the 

 alimentary canal. Taste is a visceral sense, while smell is a 

 somatic sense. While in most vertebrates the taste-organs 

 are restricted to the mouth, in some fish, such as the catfish, 

 they are distributed over the surface of the body. 



Jacobson's Organ.— Associated with the nose in land- 

 vertebrates is a pair of pouches which constitute Jacobson's, 

 or the vomero-nasal organs. Their function is doubtful, but 



