I STRUCTURE—-BRANCHIAL SAC 47 
of the branchial sac to the oesophagus, guided by a membranous: 
fold, the dorsal lamina (Fig. 21, d./), which is more or less ridged 
or corrugated, and may be armed with marginal tags or even 
replaced by larger processes (the “languets”) im some species 
of Ascidians. In the lving animal the lamina has its free 
edge curved to the right hand side in such a manner as to. 
constitute a fairly perfect tube along which the train of food 
passes. 
Branchial Sac.—Thus we have the dorsal lamina (or the 
languets) along the dorsal edge, the endostyle along the ventral 
edge, and the peripharyngeal bands around the anterior end. The 
wall of the branchial sac itself is penetrated by a large number 
of channels through 
which blood flows. Some 
of these run in one direc- 
tion and some in an- 
other, so as to form 
comphecated networks, 
which differ greatly in 
their arrangement in 
different Ascidians. Be- 
tween these blood- 
channels there are clefts 
(“ stigmata ”), the 
secondary or subdivided 
gill-slits, by means of 
which the current of 
water passes from the 
- : Fic. 21.—Antero-dorsal part of pharynx in Aseidia 
branchial sac to the mentula, x15. 67.s, Part of branchial sac; d.J, 
large external peribran- dorsal lamina; d.¢, dorsal tubercle ; p.br.2z, pre- 
: : : branchial zone ; p.p, peripharyngeal bands ; sph, 
chial or atrial cavity. aL rsh ae : 
sphincter of branchial aperture ; tm, tentacle. 
All the stigmata (of 
which there may be several hundred thousand) in the wall of 
the branchial sac are bounded by cubical or columnar epithelial 
cells, which are ciliated. These cilia, so long as the animal is 
alive, are in constant motion, so as to drive the water onwards, 
and it is this constant ciliary action in the walls of the branchial 
sac that gives rise to the all-important current of water stream- 
ing through the body. In addition to the stigmata there are 
generally one or two much larger elongated slits (Garstang’s 
