Busk on Volvox glohator. 
35 
parent, with a delicate transparent areola, apparently of gela- 
tinous matter We have now to examine more minutely the 
structure and nature of the green granules, and the further 
changes they undergo. 
Upon examination of the wall of a full-grown V. glohator 
with a sufficient magnifying power, it will be seen, upon view- 
ing the edges, as it were, of the image in the field, with the 
object so arranged as to bring the equatorial plane exactly into 
focus (fig. 5), that the green granules are, in fact, vesicular or 
semivesicular bodies of a flask-like or conical form, about 
l-3000th of an inch in transverse diameter, and placed at 
uniform distances apart. Each of them is prolonged out- 
wardly into a sort of peak or proboscis of a transparent and 
colourless or hyaline material, and from which proceed two 
very long vibratile cilia, which in close contact at first, pass 
through the parent cell-wall, upon the outer side of which 
they separate widely and perform very active movements. The 
outer cell- wall presents a minute infundibuliform depression at 
the point of exit of the cilia. It will also be observed, that 
each ciliated cell or zoospore, as it may analogically be termed, 
contains a green granular mass or masses, composed, for the 
most part probably, of chlorophyll granules and a more trans- 
parent body, which I suppose may be regarded as a nucleus, 
and derived, as it would appear, from one of the bright sphe- 
rules which have been noticed before. At an early period after 
the maturity or completion of the zoospores they exhibit a 
minute, circular, clear space, or sometimes, but I think rarely, 
more than one, which is worthy of very attentive considera- 
tion. This space is of pretty uniform size in all cases, and 
about l-9000th of an inch in diameter. It may be situated in 
any part of the zoospore, or not unfrequently in the base, or 
even in the midst of one or other of the bands of protoplasm 
connecting it with its neighbours. Its most important charac- 
ter consists in its contractility — a property already known to 
be possessed by similar spaces or vacuoles in vegetable spores. 
But what appears to me a very curious, and as yet unnoticed 
peculiarity of this contraction, consists in the fact that it is 
very regularly rhythmical. In several cases in which I have 
watched the phenomenon in question, uninterruptedly, for some 
time, the contractions or pulsations occurred very regularly at 
intervals of about 38 " to 41". In one case, however, if I was 
not misled in the observation, the interval was about twice 
this, viz., r 25". The contraction, which appears to amount 
to complete obliteration of the cavity of the " vacuole," takes 
place rapidly or suddenly, as it were, whilst the dilatation is 
slow and gradual, The interval above noted was measured 
