Chap, xii.i AUDITORY ORGANS. 461 



Among the Echiiiodermata, auditory organs 

 have only been satisfactorily observed in the deep 

 sea-dwelling holothurian Elasipoda, where they 

 are often present in large numbers, Kolga nana hav- 

 ing no less than fifty-six auditory sacs ; as in such 

 Annelids as possess them, the sacs are set close to 

 the nerve cords, and have a large number of contained 

 otoliths or concretions. 



We have more definite information as to the 

 Arthropoda and Molliisca; in the former they 

 are not always developed on the head \ Mysis, just like 

 Fabricia (among worms) with its posterior eyes, 

 showing us that, inasmuch as sensory cells are distri- 

 buted over the whole of the body, special sense organs 

 may be developed at any part, and pointing the moral 

 by having auditory organs in its terminal segment. 

 As a rule they are, as in the crayfish, developed at 

 the base of the antennules. Here the auditory sac is 

 permanently open, though the setae that protect it 

 prevent the entrance of much foreign matter ; within 

 this sac part of the wall is raised up into a ridge, and 

 the cells that form it are provided with delicate setae 

 at their free end, and with nerve fibres at their base 

 and within. The sac is filled with a gelatinous fluid 

 in which are to be found minute otoliths ; these last, 

 being set in motion by vibrations in the water which 

 strike on the guarded open slit of the ear sac, affect 

 the setae ; the setae affect the cells on the acoustic 

 ridge, and so the contained nerve fibres which are 

 in direct connection with the brain. 



While the sac of the Crustacea resembles in some 

 respects the embryonic ear of the Yertebrata, that of 

 Insects presents other points of similarity. Placed 

 on the median segments of the body or on the legs, 

 the ears of the Orthoptera are remarkable for the 

 possession of a drum or tympanum ; this is merely a 

 modification of part of the chitinous integument of the 



