EAR-SWIMBLADDER RELATION IN CLUPEOIDS 241 



4. The floor of the recessus utriculi rests on the lips of the 

 fenestra and is attached to them by delicate connective-tissue 

 strands. The utriculus does not send a diverticulum into the 

 superior chamber of the anterior osseous chamber. 



5. The upper chamber of the anterior osseous capsule con- 

 tains very sparse connective tissue and is similar in structure to 

 the perilabyrinthine spaces. 



6. The macula recessus utriculi on the floor of the recessus 

 utriculus is subdivided into three parts (anterior, middle, and 

 posterior). The anterior and middle parts of the macula are so 

 situated that they lie along the line of attachment of the floor 

 of the recessus utriculi to the anterior and posterior fenestral 

 lips, respectively. 



7. The space between the anterior and middle divisions of the 

 macula is bridged by the otolithic membrane in which the cilia 

 of the macular cells are probably embedded. 



8. The otolithic membrane originates from the cuticulum over 

 the macular cells. Probably its further development takes 

 place by the secretion of substance from the surface of the 

 macular cells. 



9. The perilabyrinthine canals form a system of spaces 

 through which changes in hydrostatic pressure of the water are 

 transmitted to the thin walls of the anterior membranous vesicle. 

 The mechanical relations are such that this transmission of pres- 

 sure causes a slight motion of the anterior and middle divisions 

 of the macula acustica utriculi, and a consequent stimulation 

 of the cells by means of the cilia embedded in the otolithic 

 membrane. 



10. The anterior and middle divisions of the macula acustica 

 utriculi constitute a receptor stimulated by changes in hydro- 

 static pressure resulting in the movement of the fish from one 

 water level to another. 



11. Impulses arising from this receptor probably originate 

 the reflex which controls the gas-regulating mechansim in the 

 swimbladder. They may also give rise to reflexes which produce 

 compensatory movements of the swimming musculature and 

 thereby maintain the fish within certain water levels. 



