132 



THE ANATOMY OF VEETEBRATED ANIMALS. 



externally by two abdominal pores. The heart presents a 

 single auricle, receiving the venous blood of the body from 

 a SWIMS venoms. There is a single ventricle, and the walls 

 of the aortic bulb contain striped muscular fibres, and are 

 rhythmically contractile, pulsating as regularly as those of 

 the auricle and ventricle. 



The interior of the bulb exhibits not merely a single row 

 of valves at the ventricnlo-bulbous apertxire, but several other 



Fig. 36. 



Fig. 36. — The aortic bulb of a Shark (Lanina), laid open to show the 

 three rows of valves, v, v, v, and the thick muscular wall, m. 



transverse rows of semilunar valves, which are attached to 

 the walls of the bulb itself, and at its junction with the 

 aorta. These valves must be of great importance in 

 giving full effect to the propulsive force exerted by the 

 muscular wall of the bulb. 



In a good many Elas^mohrancJiii there is a spiracle, or aper- 

 ture leading into the cavity of the mouth, on the upper side 

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

 aperture (which, according to the observations of Professor 

 Wyman, is the remains of the first visceral cleft of the 



