VAN NAME: COMPOUND ASCIDIANS. 
387 
cies. The muscle fibers in the mantle are comparatively weak and 
mainly longitudinal. 
The branchial aperture is obtusely six-lobed; the atrial aperture, 
which is placed at the end of a somewhat funnel-shaped tubular pro- 
jection, is situated posterior to the 
middle of the thorax in adult zooids, 
though further forward in young ones. 
It is plain-margined in young zooids, 
but often conspicuously lobed in older 
ones. This ttibe often gets broken off 
in dissecting otit the zooid from the 
common test. 
In many individuals it can be clearly 
seen that there are but eight tentacles, 
larger and smaller ones alternating, 
those opposite the endostyle and me- 
dian dorsal vessel being larger than 
any of the others. In other individ- 
uals there are additional still smaller 
ones, or 16 in all. The dorsal tubercle 
is situated close against the ganglion. 
There are three rows of stigmata, 
with a rather broad space free from 
stigmata intervening between the an- 
terior end of the thorax and the first 
row. In one individual where the stigmata could be accurately 
counted, there were 12 in the first and 11 in the second and third rows 
on each side; in another, 11 in the first two rows and 10 or 9 in the last. 
In others, 10 was the maximum in the anterior rows, though in those 
cases the zooids were perhaps not mature. The stigmata begin quite 
close to the endostyle and extend to the median dorsal vessel. At the 
ventral end of the rows the stigmata become successively shorter as the 
endostyle is approached, but at the dorsal end only the first, if any, is 
much shortened. The dorsal languets (two in number) arise from the 
transverse vessels of the left side of the sac about opposite the second 
or third stigma from the median dorsal vessel. They are short, 
curved, and directed posteriorly. 
An oblique constriction in the intestine some distance beyond the 
stomach (which is globular and smooth-walled) is generalh' noticeable. 
Text-fig. 17. — Dklemnopsis tenerum 
(Verrill). Zooid. X 45. 
