THE ELASMOBRANCH FISHES 245 



of the cornea, the slit-like pupil, the spherical shape of the 

 lens, and the glistening membrane called the tapetum, which 

 forms an inner lining to the choroid. 



The auditory organs, being imbedded in cartilage which 

 can easily be pared away with a knife, are particularly favour- 

 able objects for study. There is no middle ear or tympanic 

 cavity, as in the frog, and except for the small openings of 

 the ductus endolymphatici, the organs of hearing are completely 

 enclosed in the cartilaginous auditory capsules, and have no 

 communication with the exterior. The general structure of 

 the auditory organ is very similar to that of the frog (vol. i. 

 p. 71). The essential part of the organ is the membranous 

 labyrinth, filled with a fluid endolymph, and lodged in a 

 corresponding cartilaginous labyrinth. Between the walls of 

 the membranous and cartilaginous labyrinths is a space 

 filled with a fluid perilymph. The membranous labyrinth 

 is divisible into a utriculus above and a sacculus below. 

 The latter is a simple elongated sac, produced at its posterior 

 end into a dilatation called the lagena. The utriculus is 

 larger, and is complicated by the presence of three semi- 

 circular canals which open into it viz. the anterior and 

 posterior vertical and the external horizontal canals. Each canal 

 is swollen to form an ampulla at one end before opening into 

 the utriculus. The ampullae are at the lower ends of the 

 anterior and posterior canals, and at the anterior end of 

 the external horizontal canal. A remarkable feature in the 

 auditory organ of the dogfish is the permanent opening of 

 the ductus endolymphaticus. This tubular offset of the 

 sacculus ends blindly in the frog, but in the dogfish it 

 runs upwards and inwards towards the middle line through 

 a canal' in the skull, and on reaching the outside of the 

 skull is slightly dilated, bends first forward and then back- 

 wards, narrows to form a very fine tube, and finally opens 

 on the upper surface of the hinder part of the head by a 

 minute aperture. 



If we now review the anatomy of the dogfish in the light of 

 what we already learned of the anatomy of the frog, we shall see 

 that in almost every organ it presents a more primitive and 

 simpler structure than the latter animal. The skeleton remains 

 cartilaginous throughout life, and its early simplicity is not 

 disguised by the development of bone. The component parts 



