Note 31 — Chapter III

'There is a class of animals called Ascidians, which possess a heart and a circulation, and up to the year 1824 no one would have dreamt of questioning the propriety of the deduction, that these creatures have a circulation in one definite and invariable direction; nor would any one have thought it worth while to verify the point. But in that year M. von Hasselt, happening to examine a transparent animal of this class, found to his infinite surprise that after the heart had beat a certain number of times, it stopped, and then began beating the opposite way, so as to reverse the course of the current, which returned by-and-by to its original direction.'—Huxley's Lay Sermons, p. 95.