THE ELASMOBRANCHII. 133 



embryo), as well as from the proper branchial clefts, long 

 branchial filaments protrude, in the foetal state. These 

 disappear in the adult, the respiratory organs of which 

 ai"e flattened pouches, with transversely -plaited walls, from 

 five to seven in niimber. They open by external clefts 

 upon the sides (Sharks and Chimcera), or under-surface 

 (Rays), of the neck, and, by internal apertures, into the 

 pharynx. 



The anterior wall of the anterior sac is supported by the 

 hyoidean arch. Between the posterior wall of the first, and 

 the anterior wall of the second sac, and between the ad- 

 jacent walls of the other sacs, a branchial arch with its 

 radiating cartilages is interposed. Hence the hyoidean 

 arch supports one series of branchial plates or laminae; 

 while the succeeding branchial arches, except the last, bear 

 two series, separated by a septum, consisting of the ad- 

 jacent walls of two sacs with the interposed branchial 

 skeleton. 



The cardiac aorta, a tiimk which is the continuation of 

 the bulb of the aorta, distributes the blood to the vessels of 

 these sacs ; and it is there aerated by the water which is 

 taken in at the mouth and forced through the pharyngeal 

 apertures, outwards. 



The kidneys of the Elasmobranchii do not extend so far 

 forward as those of most other fishes. The ureters gene- 

 rally become dilated near their terminations, and open 

 by a common urinary canal into the cloaca behind the 

 rectum. 



The brain is well developed. It usually presents a large 

 cerebellum overlying the fourth ventricle, the side- walls of 

 which (corpora restiformia) are singularly folded (Fig. 37, 

 A., a) ; and moderate-sized optic lobes, which are quite dis- 

 tinct from the conspicuous thalamencephalon, or vesicle of 

 the third ventricle. The third ventricle itself is a relatively 

 wide and short cavity, which sends a prolongation forwards, 

 on each side, into a large, single, transversely-elongated 

 mass (Fig. 37, a), which is usually regarded as the result of 

 the coalescence of the cerebral hemispheres, but is, perhaps. 



