354 ZOOLOGY 



are not connected with the pharynx, but in all air-breathing 

 forms there is such connection, and the nostrils constitute the 

 normal inlet for air to enter the lungs. The sense of smell is 

 much more developed in the air-breathing vertebrates, if indeed 

 it can be said to exist at all in the aquatic animals. It is not 

 always easy to distinguish between smell and taste. The ol- 

 factory organs are more sensitive than those of taste. At any 

 rate smaller particles of the stimulating substances will arouse 

 the sense of smell than will serve to arouse taste. This makes it 

 true that smell is useful to detect chemical conditions at a greater 

 distance than is possible in taste. 



373. The Ear. There is a single pair of ears in verte- 

 brates, and these are located at the sides of the head behind the 

 eyes. The essential sensory portion of the ear (internal ear) 

 arises as an in-pocketing of ectoderm, and consists of a closed, 

 fluid-filled membranous sac which is surrounded by meso- 

 dermal structures often solid bone. Ordinarily this sac is con- 

 stricted, being thus partially separated into two irregular cham- 

 bers, the dorsal (utriculus) and the ventral (sacculus). From 

 the former arise three semicircular canals which are supplied 

 with sensory hair-cells in the epithelial lining and are looked 

 upon as being an organ to assist in detecting direction of motion 

 and maintaining balance or equilibrium. From the sacculus 

 arises an outgrowth, the cochlea, which in higher forms is well 

 developed. It becomes spiral, and is well supplied with sensory 

 cells. It is regarded as the chief auditory organ in those forms 

 possessing it. This membranous sac or labyrinth is completely 

 surrounded by cartilage or bone in fishes, and lies toward the 

 middle line from the spiracle. There is no external ear. In 

 forms above the fishes a membrane (tympanic membrane) stops 

 up the spiracle and incloses what is known as the middle ear, 

 which still communicates with the mouth by the Enstachian 

 canal. A bridge of minute bones is also formed from the tym- 

 panic membrane across the middle ear whereby the external 

 vibrations can be communicated to the internal ear. In addi- 

 tion to this, particularly among the mammals, is found an exter- 

 nal tube leading to the tympanic membrane. Expanded folds 



