AND PHYSIOLOGY, 



399 



729. lie a ring of Fishes— Among the lowest order of 

 fishes the organ of hearing is simply a sac, which is full of a 

 fluid that contains small bones or bits of bones called otoliths, 

 and the auditory nerve is distributed upon its walls. In 

 almost all fishes there is a more or less perfect form of the 

 semi-circular canals. In some fishes the swimming bladder 

 extends to membranous spaces in the cranium which are in 

 connection with an auditory apparatus. 



730. Articulates. — Among the Crustaceans hearing has 

 been observed only among the Decapods. The sense exists 

 in the Arachnoids, but no organ has been found. The same 

 is true of Insects, Annelids, and Helminths. 



731. Molluscs . — Hearing has been ascertained in Lamelli- 

 branchiata. In Cephalophora it is of a low grade : but an 

 auditory nerve and organs exist in the Cephalopoda. 



732. Radiates . — The organ of hearing exists in the Polypi 

 and perhaps the Acalephs, but has not been discovered in 

 the other Radiates. 



SENSE OF TOUCH. 



DEFINITIONS AND DESCRIPTIONS. 



733. Location of the Sense of Touch —The organ of touch, 

 with the exception of the upper surface of the tongue, is the 

 skin. This in some parts is much more sensitive than in 



729. Do fishes have tflie regular organs of hearing? What organs do they have in a 

 rudimentary state ? 730. What apparatus of hearing do articulates possess ? Do they 

 seem to hear sounds? 731-732. What is said of the sense of sound in molluscs and rad- 

 ates ? 733. Where are the organs of touch located ? 



