TAIL-KEGION OF FOLlSTOTliEMA (u! )ELI.OSTO.M a) STOUTI. 315 
ventral spinal nerve rainus^ between the in. cordis candalis 
and the myotonies, probably supplying both, though I could 
trace no branches going to the m. cordis caudalis. The 
fourth and last blood-vessel visible in this region is a vein, 
which terminates in the extreme dorso-lateral side of the left 
caudal heart, and to trace backward it passes over the m. 
cordis caudalis to the left caudal artery, which it follows for 
a short distance, and then divides into a large dorsal branch 
and a short ventral branch; the latter drains only the 
myotonies. 
There is doubtless considerable variation concerning the 
arrangement of these inter-segmental vessels in different 
individuals, as will be shown in comparing the 20 cm. to an 
85 mm. series, and also on the opposite sides of the same 
individual. In the 85 mm. embryo only three intersegmental 
vessels were noted. The first two were normal inter- 
segmental arteries and the third was a vein. The vein in 
-many respects is homologous to the second vein of the 20 cm. 
specimen ; its point of termination in the heart is not only 
much lower, but it is also much further caudad, being where 
the heart is still in embryonic condition. 
According to (xreene the caudal heart acts on the principle 
of a double force pump, the two ni. cordis caudalis contracting 
■alternately, and their rhythmic contractions are said to be 
governed by an automatic heart centre situated in the posterior 
■end of the spinal cord. 
Ventral Veno-ly mphatic Trunk (Figs. 1, 2, 4 and 4 a, 
F. F. or R. and L. V. F.). — For convenience, and in part on 
^mbryological grounds, this important canal, which collects 
the lymph and blood from the entire tail region to discharge 
it into the caudal hearts, can be divided into two portious. 
First, an anterior portion (Figs. 1, 2 and 4, F. F. or B. and 
L. V. F.), which travels along the base of the anal fin imme- 
diately below the mucous sacs. Sometimes this trunk is a 
single canal and again it is paired ; as a rule, it is a single 
vessel between two rays and paired opposite them. Hence, 
from a study of the adult alone one might erroneously take 
