EEPORT ON THE TIJNICATA. 
51 
or less elliptical or ovate, and a little less than 1 mm. in its greatest extent. The 
light mark surrounding the branchial aperture is caused by a band of opaque white 
pigment in the part of the mantle forming the branchial siphon. The anterior 
extremity of the endostyle is occasionally visible in the external view of the animal. 
The Ascidiozooids are not always placed with their long axis perpendicular to the 
surface of the colony. In fact, in the thinner parts they always lie more or less flat, 
and there is considerable irregularity in their arrangement, the systems being difficult 
to trace, and recalling somewhat the arrangement in some groups of the Didemnidse. 
The variegated or speckled appearance of the marginal regions of the colony free from 
Ascidiozooids is due partly to the presence of large numbers of vessels with their 
terminal knobs filled with black pigment corpuscles, and partly to a certain amount of 
opaque white pigment, both colours showing clearly against the semi-transparent grey 
of the test. In the greater part of its extent the large colony is thin, but in two or 
three places it has become considerably thickened, up to 4 mm. The smaller colonies 
are thin throughout ; one measures 2 cm. in its greatest extent, and the other 6 mm. 
The test is very soft and gelatinous. The matrix is perfectly transparent. The cells 
though minute are very numerous, and the vessels are abundant. Their terminal knobs 
are large, and the pigmented corpuscles which they contain are also large. The latter 
are ovate in shape and have the black pigment usually in one half only of the cell. 
They are exactly like the pigment cells of Botrijlloides perspicuuiii (see p. 47 and PI. III. 
fig. 12), except that the pigment granules in that case are brown. Besides these dark 
pigmented cells there are also in the terminal knobs a number of yellowish and of trans- 
parent corpuscles which are rather smaller but otherwise similar to the pigmented cells. 
The mantle, where free from pigment, is perfectly transparent ; but in the greater 
part of most Ascidiozooids pigment cells like those of the vessels in the test are very 
abundant, scattered through the connective tissue. Transverse muscle bands are 
present, but they are very delicate and quite colourless. The body of the Ascidiozooid 
when removed from the test is long and narrow, and the part occupied by the branchial 
sac is usually somewhat curved, with the concawity on the dorsal surface. 
The transverse vessels of the branchial sac are narrow but have each a few muscle 
fibres (PI. III. fig. 20, tr.). There are usually ten or twelve rows of stigmata in the 
branchial sac, and two or three stigmata in each mesh (PI. III. fig. 19). The area 
lying between the endostyle and the most ventrally placed of the internal longitudinal 
bars has four or five stigmata. In some sacs the stigmata are short and wide (as 
shown in PI. III. fig. 20). The endostyle is long and rather narrow, and is perfectly 
straight. 
The alimentary canal forms a more rounded mass than usual. The oesophagus is 
wide and very short (PI. III. fig. 21, cc.), and runs almost directly ventrally to open into 
the wide dorsal end of the stomach. The stomach is large, and has nearly the form of an 
