166 
THE VOZAGE OF H.M.S. CHALLENGEE. 
Tile Ascidiozooids, of which there are probably ten or twelve thousand, are 
arranged in systems which are clearly visible on the outside of the colony, on account of 
the test being decidedly more transparent than the whitish-grey bodies of the Ascidio- 
zooids. The systems are mostly circular in outline (PI. XXIII. fig. 2), like those of the 
genus Botryllus, but in some cases they are elliptical, ovate, crescentic, or more 
irregular ; they never form branched lines as in Botrylloides. Each system has a 
centrally placed common cloaca, usually of large size and distinctly visible. Many of 
these cavities are occupied by Copepoda, and the legs of the Crustaceans may usually 
be seen projecting from the openings. The systems are closely placed, and are nearly 
equally numerous over all parts of the colony. An average sized system has nine or 
ten Ascidiozooids, and is about 5 mm. in diameter. 
The Ascidiozooids, although small relatively to the size of the colony, are of 
considerable length. The anterior end, which is visible on the outside, is only about 
0'5 mm. in diameter. The middle of the thorax is the widest region, while the 
abdomen and post-abdomen are both narrow, especially the latter (PI. XXIII. fig. 3). 
The widest part of the abdomen is about its middle, as it is connected with the 
thorax by a narrower region anteriorly, and tapers posteriorly to join the genital 
region. The post-al^domen is usually very long and narrow, and the test in all parts 
of the colony is found to be penetrated by the delicate thread-like posterior ends of the 
bodies of the Ascidiozooids. Even the very centre of the colony at the widest part, 
where it is at least 2 or 3 cm. from the surface, is traversed in all directions by these 
threads. They are, like the rest of the body, of an opaque pale yellowish-white colour, 
and have an irregularly undulating course. The anterior part of the Ascidiozooid is 
placed approximately at right angles to the outer surface, but the posterior part becomes 
bent so as to run downwards towards the base of the colony. 
The test is a solid firm mass but it is not hard, and is transparent in small pieces. 
The small test cells are very numerous and are of all shapes ; some very beautifully 
branched forms occur. The test is slightly softer and less dense, and contains more cells, 
in a layer immediately surrounding the bodies of the Ascidiozooids. This is seen very 
clearly in sections stained with aniline blue, where this layer takes on the stain a little 
differently from the remainder of the test, and is thus rendered conspicuous. Many of 
the cells are very granular, but there are no opaque pigment-corpuscles present. The 
cells are particularly granular near the edges of the colony, where the test is growing 
most rapidly, and they are arranged more or less regularly in rows running parallel with 
the outer surface. 
The mantle varies considerably in the different regions of the body. Over the 
branchial sac the muscle bands are rather distant. Some transverse muscles are present, 
but the longitudinal bands are the most important. Over the abdominal viscera there 
are no transverse muscles, and the longitudinal ones are few and slight, but on the post- 
