408 
THE VOYAGE OF H.M.S. CHALLENGER. 
east of Japan, from a depth of 2300 fathoms. It agrees with the genus Bathyoncus, 
as characterised in the First Part of this Eeport, in having four-lobed apertures, simple 
tentacles, and a branchial sac with folds but no true stigmata. It differs, however, very 
greatly from Bathyoncus mirabilis, the type species of the genus, in the condition of the 
reproductive organs and in the shape of the body (see PL XLVIII. fig. 5) ; possibly it 
may have to be separated from Bathyoncus when the Deep-sea Ascidians come to be 
more thoroughly known. 
The body in Bathyoncus discoideus is fiattened antero-posteriorly to such an extent 
that the test forms merely a basal expansion attached to the surface of the manganese 
nodule, and an upper expansion which is very slightly curved and forms the anterior end 
of the body on which the branchial and atrial apertures are placed. The margin forms a 
shght spreading edge (PI. XLyill. fig. 5). The surface of the test is marked by fine 
concentric ridges or striae. The apertures are distinct (PL XLVIII. fig. 5) but not pro- 
minent. 
The test on the posterior part of the body is extremely thin, and all the inequalities 
of the surface of the manganese nodule are seen through it distinctly. The upper part 
forming the anterior surface of the body is tougher and more opaque. 
The mantle has the muscle bands running in all directions (PL XLVIII. fig. 6), so as 
to form an intricate network. 
The branchial sac shows the remarkable open structure first discovered in the genus 
Culeolus, and which seems to be associated with the abyssal zone. It has been found in 
both Simple and Compound Ascidians, but only in forms from deep water. The branchial 
folds are of various sizes. A small fold consists of two closely placed bars only (see 
PL XLVIII. fig. 8), while a large fold has as many as eight bars (PL XLVIII. fig. 9). 
The meshes between the folds are large (PL XLVIII. fig. 8). The inner edges of the 
internal longitudinal bars are thickened, but no cilia are present (PL XLVIII. figs. 8, 9, 
10). A few calcareous spicules are present in the internal longitudinal bars, but they are 
very small and difficult to see. They are visible under a high power (see PL XLVIII. 
fig. 10). The tentacles are short and stout (PL XLVIII. fig. 7). The anterior edge of 
the tentacle is thickened. 
The alimentary canal is short and of small calibre (PL XL VII. fig. 6). It lies on 
the dorsal edge of the left side of the body, behind the branchial sac. The oesophagus is 
short and funnel-shaped (PL XLVII. fig. 6). The stomach is small and globular ; it is 
smooth-walled. The intestine curves round in a wide loop so as to reach the oesophagus, 
close to which it terminates, not far from the atrial aperture. The intestine, like every 
other part of the body, is flattened, and is about 1‘5 mm. in width. It is of a light 
grey colour, and is thin-walled and almost transparent. 
The reproductive organs are situated on both sides of the body. They are in the 
form of small scattered clumps, possibly modified polycarps, most of which occur upon 
