146 
THE YOYAGE OF H.M.S. CHALLENGER. 
The Ascidiozooids are numerous and closely placed. They form little rounded pro- 
jections all over the surface of the head, but probably were not so prominent when in a 
living condition. The orange marks present in some places are caused by the alimentary 
viscera showing through ; usually they are not visible. In most of the Ascidiozooids the 
branchial aperture can be made out as a small light-coloured dot. The rest of the body 
of the Ascidiozooid is dark green. It lies obliquely in the test, its posterior end pointing 
downwards to the base of the colony as well as sloping inwards a little towards the centre 
of the head. The viscera are placed close behind the branchial sac, so there is not much 
antero-posterior elongation in the body. Most of the branchial part shows at the surface, 
and varies in length from 1 mm. to 2 mm. Some young Ascidiozooids of small size are 
present in the colony. 
The test is remarkable on account of its very limited amount, its toughness, its com- 
parative want of cell elements, and its well developed system of blood-vessels (PI. XVIII. 
fig. 8). Between the Ascidiozooids there is merely a thin layer of test, and there is not 
much more present in the centre of the head. This is probably due to a great extent to the 
shrunken condition of the colony. Most of the heads are flattened laterally, but I am of 
opinion that this is not the natural shape, but is due to distortion during the process of 
preservation. Possibly the strength and firmness of the test is also unnatural. The 
matrix is apparently structureless, and contains very few cells. The blood-vessels are 
numerous and large. They branch freely (PI. XVITI. figs. 8, 9) and have swollen termina- 
tions. In some places near the surface they are found to form by branching and 
anastomosing systems of small meshes, from which are given off numbers of swollen bulbs 
on the, side next the surface (PL XVIII. fig. 9). Nearly all the vessels contain large 
quantities of blood-corpuscles, most of which are of a green colour. The terminal knobs 
are, as a general rule, not greatly swollen (PI. XVIII. fig. 8, t.h.), being more of a club- 
shape than the ovate form found in many other Ascidians. 
The mantle has a peculiar form of musculature. The bands are thin, and in some 
places very delicate, but they are exceedingly numerous, and form a close reticulation. 
On both siphons the sphincters are very strong, and numbers of muscle bands radiate from 
under them over the neighbouring parts of the mantle, forming with the other irregularly 
running bundles of fibres a very close network. There is a good deal of pigmentation in 
the mantle, the lacunae being in many places filled with dark green blood-corpuscles. 
The branchial sac is large and well developed. It is very like the sac of some of the 
Botryllidae, and it has internal longitudinal bars. These are rather broad but very thin 
membranes, which are not placed at regular distances, and apparently vary in number. 
In a very young sac (PI. XVIII. fig. 12) three bars (the usual number in the Botryllidae) 
were found upon one side, but in some of the adult branchial sacs (PI. XVIII. fig. 10) only 
twx> bars were present upon each side. 
The meshes are irregular in size and shape, on an average each contains about four 
