230 
THE VOYAGE OF H.M.S. CHALLENGER. 
matrix is homogeneous, and is crowded with small rounded test cells. There are no 
bladder cells and very few vessels present. 
The Mantle is not strong. The muscle bands are mainly longitudinal, and are placed 
far apart. 
The Branchial Sac is small, and contains comparatively few stigmata. The trans- 
verse vessels are wide and have no horizontal membranes. The stigmata are small 
round openings regularly placed. 
Localit y . — U nkno wu. 
This species, like Atopogaster informis, is founded upon a fragment of a colony, 
preserved in absolute alcohol, which is unlike any other specimen in the collection, and 
is not labelled with any locality or date. 
It is a slice measuring about 4‘5 cm. in greatest length, 2 cm. in greatest breadth, 
and 1 cm. in greatest thickness, and has been probably cut from a colony of rounded 
form at least 5 cm. in height and 4 cm. in breadth. 
The small Ascidiozooids are irregularly scattered through a zone of test about 1 cm. 
in vddth forming the outer layer of the colony (PI. XXV. fig. 13). This outer 
layer of test is of a pale liver-colour, and the contained Ascidiozooids are opaque 
yellowish-brown. The Ascidiozooids do not show through, as the surface layer of 
test is almost opaque, consequently they do not afiect the colour of the outside of 
the colony. The central mass of test, in which there are no Ascidiozooids (PI. XXV. 
fig. 13), is of a lighter colour, being dull yellowish-grey. 
The minute rounded cells in the test are exceedingly abundant, and the dark colour 
and opacity of the outer layers are undoubtedly due to their presence. 
The whole body of the small Ascidiozooid is quite opaque (PI. XXV. fig. 13) 
although the mantle is by no means very thick. The branchial sac is in a low 
state of development (PI. XXV. fig. 14), and resembles closely the condition of 
the smallest sacs of Atopogaster informis (compare PL XXIV. fig. 13, and PL XXV. 
fig. 14). 
The alimentary canal forms a comparatively short loop and is quite opaque. The 
reproductive organs extend for a variable distance behind the intestinal loop, and in 
some cases form a comparatively long narrow post-abdomen. In sections through the 
colony the post-abdomen is seen to be occupied by numbers of mesoderm cells placed 
on both sides of a double median membranous septum. 
Although this species may resemble Atopogaster informis so distinctly in the structure 
of the branchial sac, still they are not at all closely allied. They differ greatly in the 
size of the Ascidiozooids, the condition of the test, and in various other details of 
structure described above. 
