THE LAMPREY. 21 



dorsal and left side where it comes in contact with the peri- 

 cardium, while on the right it adapts itself to the form of 

 the ventricle. 



70. The ventricle (Fig. 4, v), seen by lifting up the 

 auricle to lie towards the right side of the pericardium : it 

 has a prismoidal form, and is of firmer texture than the 

 auricle. 



71. The sinus venosus (Fig. 4, s.v), a tubular 

 chamber, passing almost vertically from the dorsal to the 

 ventral wall of the pericardium, between the auricle and 

 ventricle : it is attached to the posterior wall of the pericar- 

 dium by a thin vertical sheet of connective tissue. 



72. The ventral aorta (Fig. 4. v.ao\ springing from the 

 anterior end of the ventricle, and passing through the wall 

 of the pericardium, forwards and in the middle line, 

 between the respiratory tube above and the retractor of the 

 tongue below. 



Opposite the fourth gill-sac the ventral aorta bifurcates and is con- 

 tinued forwards as two parallel trunks. From each of these are given 

 off four afferent branchial arteries, the first supplying the first demi- 

 branch (anterior wall of first gill-sac), the second, the second and third 

 demibranchs (posterior wall of first and anterior wall of second sac), 

 the third, the fourth and fifth demibranch, and so on. The azygous 

 posterior portion of the ventral aorta also gives off four afferent arteries, 

 of which the first three supply each two contiguous demibranchs (i.e. 

 one entire gill), and the fourth goes to the fourteenth or last demibranch. 

 Each afferent artery is thus primarily related to a gill ( 54) and not 

 to a gill-sac. 



73. The inferior jugular vein (Fig. 4, i.ju) situated just below 

 the retractor of the tongue and above the median ventral cartilage of 

 the branchial basket. It brings the blood from the lower part of the 

 head to the sinus venosus, and is best made out by making a small 

 aperture in the latter and passing a probe forwards. 



74. The dorsal aorta (Figs. 4 & 6, d.ao\ immediately 



