DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 885 



in a fan-like manner into the caudal expansion, the whole having a somewhat semi- 

 circular outline, since the dorsal and ventral branches are less prominent than the 

 median. A considerable margin of the tail is devoid of vessels. A downward loop is 

 usually formed by the vessels just described, which enter the vein, and various inoscu- 

 lations also occur terminally. The terminal, dorsal, and ventral series present 

 slight irregularities, the dorsal loops especially being broader and less definite. The 

 venous twigs from the arteries just alluded to pass forward in a similar direction to the 

 two large venous trunks running along the posterior border of the hypurals. In regard 

 to the axis of the body, these veins, as well as arteries, lie at an angle of about 45°, 

 the ventral edge, moreover, having twigs which curve even further forward. By the 

 union of the foregoing venous ramifications, the great caudal is formed. It lies beneath 

 the aorta, and follows the same curved course as the artery. 



The circulation in the tail of the wolf-fish exhibits, at this advanced stage, a com- 

 plexity similar to that in the salmon two or three days old, and the contrast between 

 their rate of development is thus very great (conf. PI. XX. fig. 1, wolf-fish newly 

 hatched, with PI. XXVIII. fig. 1, salmon newly hatched). It is unlikely that the 

 development of the former in the egg is much more rapid than that of the latter, yet 

 the wolf-fish on being hatched presents only a few loops of vessels below the straight 

 notochord, while the salmon has attained a complexity of organisation in regard to the 

 vessels and the curvature of the notochord only reached by the former two months later. 

 Moreover, the nearly horizontal direction of the median vascular loops in the salmon is in 

 contrast to the obliquely downward direction of those in the wolf -fish. It must, however, 

 be remembered that the influence of altered circumstances (e.g., those in the Laboratory) 

 upon the eggs of Anarrhichas is unknown : yet other eggs with thick capsules do not 

 seem very readily affected by such external conditions. 



The size of the arterial trunks in the tail of the salmon seems to be smaller in com- 

 parison with the veins than in the wolf -fish, and, moreover, the former has a large venous 

 sac (comparable to the caudal heart of the eel) at the upward bend of the notochord, 

 forming a large ovoid dilatation sloped upward and backward, and with a siphonal bend 

 dorsally, and a slight contraction ventrally (PI. XXVIII. fig. 1, cvs). No pulsation is ever 

 present in it, except the impetus from the pulsations of the arteries and from the action of 

 the auricle. Stasis very readily affects this sac. In the somewhat older larva as much 

 blood passes by the vessels beneath as through the sac in these circumstances. The sac seems 

 to diminish rapidly, for it is indistinct on the thirteenth day, though the increased amount 

 of pigment renders accurate observation difficult. When about a week old, a secondary 

 enlargement in the form of a rounded sac sometimes occurs just behind the former. 



In the more advanced condition, as at the end of March, the vessels in the tail of Anar- 

 rhichas (PI. XXII. fig. 2) have become much more elongated, and more definitely arranged. 

 Further, instead of the obliquely downward direction of the main vessels, the whole form 

 a fan-shaped arrangement, the median being horizontal, as in the salmon on the first day. 



The two main branches which respectively leave the aorta and join the caudal vein 



