THE LAMPREY. 17 



be formed by the bifurcation of a single trunk, the caudal vein (Fig. 

 8, c.v\ which may be traced to the end of the tail, lying to the ventral 

 side of the caudal artery (Fig. 8, c.a], which is the direct continua- 

 tion of the dorsal aorta. 



FIG. 8. Petromyzon marinus. Transverse section through caudal 

 region, including the second dorsal fin (nat. size). 



c.a, caudal artery : c.v, caudal vein : f.r, fin rays : f.t, f.t', fibrous 

 tissue enclosing neural canal : m, body-muscles : my, myelon : nc, 

 notochord : n.ca, neural canal. 



IX. Place the fish once more with the left side up- 

 wards ; dissect away the dorsal and ventral muscles 

 of the branchial region ( 34), and, if the skeleton 

 has not been examined, make out the various 

 cartilages of the branchial basket ( 18) : then 

 remove the latter, as well as the muscular and 

 connective tissues obscuring the gill-sacs (53): 

 get the latter well cleaned, open one or two of 

 them and note : 



53. The gill-sacs (Figs. 4 br, and 6), seven in number, 

 arranged obliquely, each being inclined, from its inner side, 

 backwards and outwards. They are compressed from 

 before backwards, separated from one another by strong 

 fibrous and muscular partitions, and marked externally with 

 parallel horizontal lines, which mark the position of 



