264 



AN INTRODUCTION TO ZOOLOGY 



SE 



only, and the same is true, too, of its histological structure. The 

 eyeball itself is almost hemispherical, as it is much less curved on 

 its external side than in vertebrates generally, and the cornea itself 

 is practically flat instead of being curved. The lens is approximately 

 spherical and not bi-convex as in the frog. This fact in conjunction 

 with the flattened cornea causes the anterior chamber to be relatively 

 small, and such a combination of characters is found generally in 

 fishes. Lastly, between the pigment layer of the retina and the 

 choroid coat there is present in the dogfish a peculiar membranous 

 layer which, over the posterior region of the eyeball, possesses a 

 curious bright, silvery metallic lustre. It is known as the tapetum, 

 and presumably causes the reflection of a certain amount of light. 

 Vision in the dogfish is apparently much less keen than in higher 

 animals. 



Ear. 



in Scyllium and fish in general the ear is less complex than 

 in the land-dwelling vertebrates, and it consists only of the mem- 

 branous labyrinth or internal 

 ear, and we find nothing to 

 correspond with the middle 

 ear, i.e. the tympanic mem- 

 brane and cavity as it exists 

 in Rana. On account of its 

 larger size and greater acces- 

 sibility the labyrinth is more 

 easily studied in the dogfish 

 than in the frog. It lies em- 

 bedded in the cartilaginous 

 olfactory capsule on the outer 

 walls of which, as we have 

 already noted, are three ridge? 

 marking the position of the 

 canals within. The vestibule 

 is a very thin-walled laterally 

 compressed sac, distinctly 

 marked off into two portions ; 



A., anterior vertical semicircular canal; Am., the Upper, Or UtricuhlS, is 

 ampulla ; D.E., ductus endolymphaticus ; H., 

 horizontal semicircular canal ; L., lagena ; P., 

 posterior vertical semicircular canal; S., saccu- 

 lus ; S E., saccus endolymphaticus ; U., utriculus. 



FIG. 86. -Diagram of ear of primitive 

 Craniate, adapted from Wiedersheim. 



elongated and triangular in 

 shape, the lower, or sacculus, 

 is a smaller more rounded sac 

 in open communication with it. The semicircular canals bearing 

 ampullae, and coming off from the utriculus are well developed 

 and present certain peculiarities. The anterior and posterior canals 



