50 ASCIDIANS CHAP. 
tion pass along the heart from end to end, first for a certain 
number of beats in one direction, and then, after an interval, in 
the other. If a small or young <Ascidia be placed alive, left 
side uppermost, in a watch-glass or small trough of sea-water, 
and examined with a low power of the microscope, the heart will 
be readily seen near the posterior end of the transparent body. 
It will be noticed that the “ beating” looks hke successive waves 
of blood pressed through the tubular heart from one end to the 
other by its contractions. After watching the waves passing, let 
us say, from the right hand end of the heart to the left for about 
a minute and a half (perhaps 60 or 80 to 100 beats), it will 
be seen that they gradually become slower and then stop 
altogether. But after seven or eight seconds a faint wave of 
contraction will start from the /ef/¢ end of the heart and pass 
over it to the right; and this will be followed by larger ones 
for a minute and a half, and then again a pause will occur and 
the direction change. It has been suggested that the cause of 
this remarkable reversal may possibly be that the heart being on 
the ventral vessel, which is wider than the corresponding dorsal 
trunk, pumps the blood into either the lacunae of the branchial 
sac or those of the viscera in greater volume than can possibly 
get out through the smaller branchio-visceral vessel in the 
same time, the result being that the lacunae in question soon 
become engorged, and by back pressure cause the stoppage, and 
then reversal of the beat. The absence of any valves in the 
heart to regulate the direction of flow obviously facilitates this 
alternation of the current. 
The larger channels through which the blood flows may be 
lined with a delicate endothelium, but the smaller passages are 
merely spaces in the connective tissue. The heart, although 
anatomically a “ventral vessel,” runs in the main dorso-ventrally. 
The blood-channel leaving the ventral end of the heart is the 
“ branchio-cardiac vessel” (Fig. 23, b.c). This gives off a branch 
which, along with a corresponding branch from the “cardio-visceral” 
vessel (c.v) at the other end of the heart, goes to the test, and then 
runs along the ventral edge’of the branchial sac as the branchial 
aorta (b.a), external to the endostyle, communicating laterally 
with the ventral ends of all the transverse vessels of the branchial 
sac. The cardio-visceral vessel (Fig. 25, c.v) after giving off its 
branch to the test breaks up into a number of sinuses which 
