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XXIX. On the Anatomical Characters of a remarkable form of Compound Tunicata. 
` By Joux Denis MACDONALD, F.R.S., Assistant-Surgeon of H.M.S. * Herald,’ com- 
manded by Captain H. M. DEXHAM, R.N., F.R.S. Communicated by GEORGE Busk, 
Bigy FRS. BLS ot 2 ! 
. Read February 17th, 1859. . ` 
ON examining an encrusting form of Compound Tunicata, taken from the ship’s copper 
while refitting in Sydney Harbour, I was surprised to find that each little cluster of 
viscera was surmounted by two similarly-constructed branchial chambers or thoraces, as 
though two zooids had been combined together. | 
‘ Each branchial chamber was supported on a narrow pedicle, and both pedicles arose 
from one short trunk, which suddenly expanded into the abdomen, while little gemma- 
tions were frequently seen clustering near its base. 
About four delicate and unbranched tubules, with a dilated and glandular extremity 
(derived from that part of the mantle which invests the viscera), extended themselves into 
the connecting substance. | 
A stout endostyle oceupied the dorsal region, and the branchial network exhibited 
three or four principal transverse bars, intersected at right angles by the more numerous 
longitudinal nervures. The orifice of entry was guarded by a cirelet of six broad and 
short tentaculiform processes—organs which are so often mistaken, in other cases, for 
the true tentacula; and there being no proper atrium, the anus opened directly upon the 
surface near the middle of the ventral aspect. The existence of a superficial common 
cloacal system was clearly indicated by the low position of the rectum, and the absence of 
pigment-cells within a circumscribed space on the fore-part of the body. The actual 
disposition of this system, however, I have not succeeded in determining, on account of 
the peculiar delicacy of the connecting substance. ser 
A small spur:like appendage or caudex was sometimes distinctly observed, projecting 
from the dorsal surface of the pedicles, just below the branchial chamber. i 
. The viscera formed a large subglobular mass, in which a voluminous stomach, testis, 
and ovarium were plainly discernible. In the specimens examined, the diameter of the 
ova, visible in the ovarium, far exceeded that of the pedicles, through which, according 
to the present view of the subject, they were destined to pass; moreover, numerous ovo, 
scarcely further advanced than those within the ovarium, were scattered through the 
connecting substance, in which they were perfectly enclosed. The ova of this genus, like 
those of the larger solitary Ascidians, were invested with a stout chorion, supporting a 
beautiful epithelial pavement, and containing å dull amber-coloured or reddish yelk. 4 
The process of yelk-cleavage was easily traceable in a selec series of ova ; and where 
that of differentiation had commenced, the vitelline mass appeared to be encircled almost 
completely by a long and gradually tapering tail, while three short — — 
m an opposite point. In more advanced examples, the transparent polygo a " : ^ 
cluding the true embryonic structures, formed an oval tadpole-like body, from the delica 
