ASCIDIA. 53 
so that the stigmata form more or less complete spirals. 
In some species of Ascidia, and other Ascidians, the inter- 
stigmatic vessels are inserted into the transverse ones in un- 
dulating in place of straight lines, 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 device for increasing the surface is known as 
“minute plication,” and is seen well in Ascidia mentula 
(Pl. IV., figs. 1 and 2). In some cases, again (Cynthide), 
the whole wall of the sac is pushed inwards at intervals 
to form large folds visible to the eye. 
The intersections of the internal longitudinal bars with 
the transverse vessels divide up the inner surface of the 
branchial sac into rectangular areas called ‘‘meshes”’ (PI. 
IV., fig. 2). One such mesh, containing eight stigmata in 
a row, is seen in fig. 3, Pl. Il. The internal longitudinal 
bars bear hollow papille at the angles of the meshes, and 
occasionally in intermediate positions. There are frequently 
horizontal membranes (fig. 3, B. h.m.) attached to the 
transverse vessels between the papille, and the transverse 
vessels may be of two or more sizes arranged symmetri- 
cally. There are many ‘‘ connectives”? running from the 
outer wall of the branchial sac to the body-wall, and 
allowing the blood in the transverse vessels to communi- 
cate with that in the sinuses outside (PI. II., fig. 2, con.). 
In an adult medium-sized Ascidia mentula there are 
about 150 transverse vessels on each side of the branchial 
sac, and at least 80 internal longitudinal bars, making by 
their intersection 12,000 meshes. The average number 
of stigmata in a mesh is, in this species, six to eight. So 
there may be as many as 96,000 stigmata present on each 
side of the sac, nearly 200,000 in all. Probably these 
numbers are greatly exceeded in large specimens, such as 
that shown on Pl. I. These stigmata are to be regarded as 
