MOLLUSOA— ANNELIDES. 



87 



the fact that one species, Tritonia arhorescens, emits 

 certain sounds, doubtless intended to be heard by its 

 fellows. 



The ciliae contained in the auditory vesicle are some- 

 times short, and scattered over 

 the general surface, as in Unio 

 (Fig. 55) ; sometimes long and 

 borne on papillary projections, as 

 in Carinaria and Pterotrachea * 

 (Fig. 56), where also there are 

 certain special cells, supposed to 

 act as buffers or dampers. The 

 otolithe is sometimes single, and 

 nearly spherical, as in Acephala 

 and Heteiopoda, and consists of calcareous matter with 

 an organic base ; in the Gasteropods, Pteropods, and 



m 



Fig 



55. — Auditory organ of 

 Unio (after Leydig). a. Nerve ; 

 5, cells ; c, cilise ; d, otolithe. 



Fig. 56. — Auditory organ of Pterotrachea Friderici (after Claus). Na, Auditory nerve ; 

 c, central cells ; d, supporting plate ; b, outer circle of auditory cells ; a, ciliated 

 cells. 



some Annelid es (Arenicola, Amphicora) they are 



* Claus., " Ueber deu Acoust. App. im Gehoroigaiie der Hetero- 

 poden," Arch, fur Mic. Anat, 1878. 



