^3 



beats. If a young individual of a small transparent species, 

 such as Ascldla virginea, be placed alive, left-side upper- 

 most, in a watcb-glass 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 eitherthe 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 lacunse 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, 



