48 ASCIDIANS CHAP. 
pharyngo-cloacal slits) placed close to the dorsal lamina and 
leading direct to the cloaca. 
Fig. 22 shows a small part of the wall of the branchial sac, in 
which it may be seen that the bars containing the blood- 
channels are arranged in three regular series:—(1) The “ trans- 
verse vessels” which run horizontally round the wall and open 
at their dorsal and ventral ends into large median longitudinally 
running tubes, the dorsal blood-sinus (or “ dorsal aorta”) behind 
the dorsal lamina, and the ventral blood-sinus (or “ branchial 
aorta”) beneath the endostyle; (2) the fine longitudinal or 
“interstigmatic vessels” which 
run vertically between adjacent 
transverse vessels and open into 
them, and which therefore bound 
the stigmata; and (3) the “in- 
ternal longitudinal bars” which 
run vertically, in a plane internal 
to that of the transverse and 
fine longitudinal vessels. These 
bars (Fig. 22, 2.2) communicate 
with the transverse vessels by 
short side branches where they 
cross, and at these points are 
EN coer ae ee branchial sac of prolonged into the cavity of 
Ascidia, seen A, from inside; B, in : 
horizontal section. ¢.d, Connecting the sae in the form of hollow 
such; rs hovontat membrane; papillae, In some Ascidians 
al long ars ; 1.2, 
stigmatic vessels ; p, p’, papillae ; sg, (e.g. Corella and most of the 
stigmata ; ¢r, transverse vessels. Molgulidae) the interstigmatic 
vessels are curved so that the stigmata form more or less com- 
plete spirals (see Figs. 35 and 41). In some species of Aseidia, 
and other Ascidians, the interstigmatic vessels are inserted into 
the transverse vessel in an undulating course in place of the 
straight line seen in Fig. 22, B, l.v, the result being that the 
stigmatic part of the wall of the branchial sac seems to be folded 
or thrown into microscopic crests and troughs. This is known as 
“minute plication.” In some cases, again (Cynthiidae), the 
whole wall of the sac is pushed inwards at intervals to form 
large folds visible to the eye (see Fig. 36, A and B). The 
intersections of the internal longitudinal bars with the trans- 
verse vessels divide up the inner surface of the branchial 
