138 
THE YOYAGE OF H.M.S. CHALLENGEE. 
The colour on the upper surface varies from yellowish-grey to various shades of 
hrownish-grey. It is lightest on the edges where composed of test only. The bodies of 
the Ascidiozooids show through indistinctly (PL XIX. fig. 1) as opaque light coloured 
rounded spots.^ The common cloacal apertures are fairly numerous but not conspicuous. 
They are irregularly scattered. The under surface of the colony is rather lighter 
coloured than the upper, but is more opaque-looking, probably on account of the absence 
of the very transparent glistening superficial layer of test found on the upper surface. 
The Ascidiozooids are placed nearer to the lower surface, and their white calcareous 
capsules show through more distinctly than they do on the upper surface. 
The Ascidiozooids seem to be to a certain extent arranged in systems of five or six 
together, but these are by no means regular. Although the branchial apertures are all 
on the one surface of the colony, and the posterior ends are all placed nearest to the 
other surface, still the Ascidiozooids are not regularly arranged perpendicularly to the 
upper surface ; consequently, in a vertical section of the colony (such as PL XIX. fig. 4), 
they may be cut at various angles. The shape of the Ascidiozooid is remarkable (PL XIX. 
figs. 4, 5), but it must be remembered that it is mainly due to the presence of the thick 
calcareous capsule which encloses the posterior end, and which, although it adheres closely 
to the Ascidiozooid as dissected out from the colony, really belongs to the common test in 
which it is produced. 
The test varies somewhat in structure in the different regions of the colony. Figure 2 
shows a section extending from the outside (on one of the margins) for a considerable 
distance inwards. The outer layer is seen to be free from bladder cells, and to have 
scattered in the matrix merely the ordinary rounded fusiform and stellate cells. These 
are comparatively large and numerous. Then as the section is traced inwards, bladder 
cells make their appearance and rapidly become very abundant, — even more abundant in 
some places than appears from the figure. They are large, spherical or ellipsoidal in 
shape, and have distinct nuclei adhering to one part of the wall. In the narrow bars of 
test matrix left between these bladder cells, the small fusiform and stellate cells are still 
found. This is the region of the test where the bladder cells are most abundant. 
Further in towards the centre of the colony they become less numerous, although they 
are always fairly abundant. The small test cells continue to be thickly scattered in all 
parts of the colony (PL XIX, figs. 2, 3, t. c.). 
In the deeper parts of the test, away from the margins, are found groups of rounded 
cells with very large distinct nuclei (PL XIX. fig. 2, t. c/), and occasionally a small calcareous 
spicule may be found in the centre of such a clump. These indicate the mode of origin 
of the remarkable calcareous capsules enclosing the Ascidiozooids. The small fusiform or 
branclied test cells may apparently in this genus either develop into 'large bladder cells, 
^ The upper surface of the single specimen presents rather a curious appearance in some parts from small air- 
huhbles having gained access to spaces (the cloacal cavities) under the surface layer of test. 
