120 



The Organs of Sense 



(called Ostariophysi) the two vestibules are joined in a median 

 sac (sinus impar) in the substance of the basioccipital. This 

 communicates with two cavities in the atlas, which again are 

 supported by two small bones, these resting on a larger one 



FIG. 88. Brook Lamprey, Lampetra wilderi Jordan and Evermann. (After Gage.) 



Cayuga Lake. 



in connection with the front of the air-bladder. The system of 

 bones is analogous to that found in the higher vertebrates, but it 

 connects with the air-bladder, not with an external tympanum. 

 The bones are not homologous with those of the ear of higher 

 animals, being processes of the anterior vertebras. The tym- 

 panic chain of higher vertebrates has been thought homologous 

 with the suspensory of the mandible. 



The otoliths, commonly two in each labyrinth, are usually 

 large, firm, calcareous bodies, with enamelled surface and peculiar 



FIG. 89. European Lancelet, Branchiostoma lanceolatum (Pallas). 

 Parker and Haswell.) 



(After 



grooves and markings. Each species has its own form of otolith, 

 but they vary much in different groups of fishes. 



In the Elasmobranchs (sharks and rays) and in the Dipnoans 

 the ear-sac is enclosed in the cartilaginous substance of the skull. 

 There is a small canal extending to the surface of the skull, ending 

 sometimes in a minute foramen. The otoliths in these fishes are 

 soft and chalk-like. 



