THE ELAFMOBRANCHII. 



117 



In a good many Elasmohranchii there is a spiracle, or 

 ai)erture leading into the cavity of the mouth, on the upper 

 Bide of the head, in front of the suspensorium. From this 

 aperture (which, according to the observations of Prof. Wy- 



FiG. 86. — The aortic bulb of a Shark (LanvncC)^ laid open lo show the three rove of valves, 

 », w, w, and the thick muscular wall, m. 



man, is the remains of the first visceral cleft of the embryo), 

 as well as from the proper branchial clefts, long branchial fila- 

 ments protrude, in the foetal state. These disappear in the 

 adult, the respiratory organs of which are flattened pouches, 

 with traversely-plaited walls, from five to seven in number. 

 They open by external clefts upon the sides (Sharks and Chi- 

 mcera), 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 

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

 the anterior wall of the second sac, and between the adjacent 

 walls of the other sacs, a branchial arch with its radiating car- 

 tilages is interposed. Hence the hyoidean arch supports one 

 series of branchial plates or laminae; while the succeeding 

 ))ranchial arches, except the last, bear two series, separated by 

 a septum, consisting of the adjacent walls of two sacs with the 

 Interposed branchial skeleton. 



The cardiac aorta, a trunk 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, put- 

 ward. 



