348 
THE VOYAGE OF H.M.S. CHALLENGER. 
if it was an isolated form it would be extremely difficult if not impossible to determine 
whether it should be regarded as a Simple or a Compound Ascidian. It is, however, 
so closely allied to Synstyela incrustans and other members of the family Polystyelidse, 
that it becomes an easier matter to trace its connection with true Compound Ascidians 
than with true Simple Ascidians, and therefore I have placed it in its present position 
amongst the Polystyelidse. Its affinities will be further discussed in the General 
Summary at the end of the Eeport. 
The masses of test containing several Ascidiozooids are each of various forms (see 
PI. XLVI. figs. 1-4), and are connected with one another by narrow bands of test. 
The larger colonies (PI. XLVI. fig. 1) consist of a very large number of these masses 
united by their stolons to form a very irregular mass clinging closely to some foreign 
body. The smaller colonies consist of several of the little masses, or of a few independent 
Ascidiozooids joined by the usual stolons (PL XLVI. figs. 2, 3, 4). As may be seen 
from figures 2 and 3, the masses differ considerably in their size. The average size is 
2 or 3 mm. in diameter. The side branches given off from the stolons are frequently 
thickened at their ends so as to form small knobs (PI. XLVI. figs. 1-4). The whole of 
the outer surface of the colony is slightly incrusted with fine black sand grains, otherwise 
it is of a dull but fight grey, and fairly smooth. 
The Ascidiozooids vary in size from about 1 mm. up to 8 mm. antero-posteriorly. 
The larger ones are generally independent of their neighbours, and have a striking 
resemblance to individuals of some of the smaller species of Styela and Polycarpa. The 
apertures in these large Ascidiozooids are placed on long tapering siphons. They are 
square when open and cross-slit when closed. 
The test is relatively small in amount in this species. Where it covers the body of an 
Ascidiozooid it is a thin tough layer, slightly sandy on its outer surface, and perfectly 
smooth and glistening internally. The Ascidiozooid can readily be shelled out from its 
coating of test, as the mantle does not adhere to it very closely. The stolons are entirely 
formed by test. The vessels are not numerous, but they are present in all parts of the 
colony. One or more may always be found in the stolon running longitudinally, and 
occasionally giving off lateral branches which may be long or short (PI. XLVI. fig. 5, t. h.). 
The terminal knobs are short and globular. The ectoderm cells on the surface of the 
vessels are very distinct (PI. XLVI. fig. 5, v.). 
The branchial and atrial sphincters are wide. They extend over nearly the whole of 
the elongated siphons. Both longitudinal and transverse muscle bands are found over 
the mantle generally ; they form a close network (PL XLVI. fig. 8). 
The branchial sac has a single rudimentary fold upon each side. It lies close to 
the dorsal lamina, and consists of four closely placed internal longitudinal bars. It is 
separated from the dorsal lamina by a single row of very wide meshes, each of which 
contains about a dozen stigmata. The transverse vessels and the internal longitudinal 
