644 JULIAN 8. HUXLEY 
work. As is well known, the social Ascidians reproduce 
asexually by means of buds given off at intervals from creeping 
branched stolons; but whilst in Clavellina the zooids may 
reach two inches, in Perophora the maximum length is only 
about one-quarter of an inch, and the span of life is probably 
limited in proportion. The branching and budding of Pero- 
phora is also much easier to follow, the stolons often growing 
in a straight line for a considerable distance, giving off buds 
at regular intervals. It is thus easy to trace a sequence from 
young to old individuals in Perophora, but hard in Clavellina. 
In Perophora it is also possible to isolate single zooids of any 
age by cutting the stolon midway between the neighbouring 
zooids on either side; and in such preparations the piece of 
stolon is of the same order of magnitude as the zooid, while in 
Clavellina the volume of the stolon is quite negligible in pro- 
portion to that of an adult, a half-grown, or even a quarter- 
grown zooid. 
Such preparations we may call stolon-zoo0id systems. 
They are composed of two very distinct parts. The stolon 
is very simple : it consists of a thin external test-layer surround- 
ing a single-layered tube of flattened ectodermal epithelium, 
which in its turn is divided into two by a horizontal partition 
composed of two very thin endodermic epithelia flattened 
together to form a single sheet ; the space between ectoderm 
and endoderm contains blood, with numerous cells of several 
different kinds. At either end of the stolon the partition 
stops short, so that the blood can circulate from one half-tube 
to the other. It is normally kept in motion by the heart-beat 
of the zooid, which, as in all Ascidians, undergoes a periodic 
reversal of direction. The cut surface of test and ectoderm 
soon heals over. In a healed preparation the ectoderm at 
either (cut) end of the stolon is more or less cuboidal, and 
presents the appearance of undifferentiated tissue. 
The zooid, on the other hand, is of high organization, con- 
taining, as it does, heart, stomach and intestine, elaborate 
branchial apparatus, nervous, muscular, and exeretory systems, 
and hermaphrodite reproductive organs. It is also highly 
