ASCIDIA. 59 
beats. Ifa young individual of a small transparent species, 
such as Ascidia virginea, be placed alive, left-side upper- 
most, in a watch-elass 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 then be-noticed that the ‘“‘ beating” looks like succes- 
sive waves of blood, which are pressed through the tubular 
heart from one end to the other by the contractions of the 
muscle fibres. After watching the waves passing, let us 
suppose, from the dorsal end of the heart to the ventral, 
for about a minute and-a-half or two minutes, it will be 
seen that they gradually become slower and then stop 
altogether. But now, after several seconds, a faint wave 
will start from the ventral end of the heart and pass over 
it to the dorsal; and this will be followed by larger ones 
for perhaps a minute or two, and then again a pause will 
occur and the direction change. So that we may say, the 
heart of the Ascidian beats first in one direction and then 
in the other; and the reversal of the blood current takes 
place every minute or two. There are generally rather 
more beats in the dorso-ventral than in the opposite 
direction, but there is considerable irregularity. The 
numbers are usually between 60 and 80. | 
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, it pumps the blood 
into either the lacunee 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 lacune in question will 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. 
