182 
THE VOYAGE OF H.M.S. CHALLENGEK. 
elliptical in outline, and have the Ascidiozooids arranged in a circle around them 
(PI. XXV. fig. 1), consequently the systems resemble those of the genus Botryllus in 
outline. Parasitic Crustaceans were found in some of the cloacal cavities. The anterior 
ends of the Ascidiozooids are seen on the upper surface of the colony as small round 
spots, and the bodies are visible on the sides of the colony in the form of pale yellow 
streaks. In sections the Ascidiozooids are seen to be equally numerous in all parts of 
the colony. 
The test is very tough and solid, and is of the same colour and consistence through- 
out. In some places the matrix is delicately fibrillated. The test cells are more 
abundant in the outer layer than elsewhere ; they are mostly fusiform, and are arranged 
with their long axis parallel to the surface. They are nearly all granular and opaque, 
while the surrounding matrix is clear and transparent. 
The longitudinal muscle bands in the mantle are very regular, but narrow ; on the 
post-abdomen they are less regular ; they are broader, but placed further apart, on the 
thorax ; and are almost absent on the abdomen. The branchial aperture is six-1 obed. 
The branchial siphon is long and narrow, and the sphincter is well developed. In some 
parts of the thorax the muscle bands are rather irregular in their arrangement and 
anastomose with one another. 
There are about twelve rows of stigmata in the branchial sac. The transverse vessels 
are strong, and are provided with double muscle bands (PI. XXV. fig. 2, m.f.). The 
anterior band of one vessel is connected at the dorsal and ventral edges with the posterior 
band of the vessel in front, and the posterior band of the first vessel is connected at the 
edges with the anterior band of the vessel behind, so that a continuous band of muscle 
fibres encircles each row of stigmata (see PL XXV. fig. 2). The effect of the contraction 
of these muscles of the branchial sac would probably be to corrugate the stigmata and 
interstigmatic vessels and so diminish the size of the sac as a whole. Probably this action 
takes place when the mantle contracts and water is forced out through the branchial and 
atrial apertures simultaneously. The stigmata are usually equal in breadth to the fine 
longitudinal vessels. The ciliated cells are large and distinct. 
The dorsal languets are large and of an elongated triangular form. They are 
numerous and closely placed. The endostyle is large and conspicuous. The tentacles 
are of two sizes placed alternately ; they are large. The nerve ganglion is ellipsoidal 
in shape and of large size. 
The oesophagus is of moderate length and runs directly backwards to open into the 
large ellipsoidal stomach (PL XXV. fig. 3). The wall of the stomach is folded so 
as to form a number of detached knobs with thickened epithelium projecting from the 
surface. This structure recalls the condition of the stomach in Morchellioides affinis, 
but in the present case the knobs or caeca are more numerous and are less distinctly 
placed in rows (PL XXV. fig. 3, st.). In a transverse section of the stomach 
