64 
THE VOYAGE OE H.M.S. CHALLENGER. 
bright yellow, the cloaca! apertures are whitish, and the surrounding area is grey, while 
the small intervals left here and there between the systems are of a bluish-grey tint. 
The systems are very numerous, and are so closely placed over the whole colony that 
very little of the test is visible, and here and there it becomes a little difficult to 
determine where the different systems end. The systems are all nearly circular in form, 
and contain from eight to fourteen Ascidiozooids each ; the general numbers are nine, ten, 
and eleven (see PI. IV. fig. 1). 
The part of the Ascidiozooid seen from the surface is of ovate form, with the narrower 
end pointing towards the common atrial aperture or centre of the system. It is on an 
average about 1’5 mm. long and 1 mm. broad, but in some places longer Ascidiozooids 
occur, measuring up to 2 '5 mm. in length. 
Under a low power of the microscope the branchial apertures, which are just 
visible to the unaided eye, are seen to be bounded by a rather wide opaque white 
band, the sphincter muscle, while a much narrower white line, the peripharyngeal band, is 
in most cases visible further out (compare PI. IV. fig. 2). The anterior part of the endostyle 
and the nerve ganglion are also usually visible; no vessels are seen in this surface view. 
The common cloacal apertures are Amry clearly visible to the naked eye, all over the 
colony. They are all more or less widely open, and are bounded by distinct thickened 
whitish rims. They are circular or elliptical in shape, and vary in size from 0*5 mm. to 
2 mm. in diameter (PI. IV. fig. 1). 
A section through the colony shows the great thickness of the test. The Ascidiozooids 
occupy merely a thin layer, of a yellowish colour, extending from 1 mm. to 1‘5 mm. 
inwards from the surface ; the rest of the thickness of the colony is formed of the soft 
grey semi-transparent test. At the lower end of the colony, near the point where the 
incrusted Zoophyte Avas attached, there is a small tapering portion of test free from 
Ascidiozooids. This is the only place where the terminal knobs of the vessels are visible 
in surface view. They are fairly numerous, but being small and of an opaque grey tint, 
they are by no means conspicuous. In the superficial layer of test over the colony 
generally, vessels are present, but they are small and their terminal knobs are of small size. 
This rather feeble development of vessels in the test of this species seems to be due simply 
to want of room in the superficial part owing to the large number of Ascidiozooids. 
The mantle musculature is irregular and of fair strength. The atrial siphon is 
prolonged into a tube of considerable length and having a rounded termination (PI. IV. 
fig. 3 at.). Its walls, which are simply a continuation of the mantle, are provided with 
both longitudinal and transverse muscle bands. The latter are more abundant and 
stronger than the former. The branchial aperture is very small, and the sphincter is 
fairly strong (PI. IV. fig. 2, sjjh.). 
The branchial sac seems stronger than is usual in the genus Botrylloides. In one sac 
examined there were ten rows of stigmata, but in some of the others there are probably 
