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XXIX. On the Anatomiccd Characters of a remarkable form o/" Compound Tunicata. 

 By John Denis Macdonald, F.B.S., Assistant- Surgeo7i of H. M.S. 'Herald,' com- 

 manded by Captain H. M. Denham, R.N., F.R.S. Communicated by George Busk, 

 Esq., F.R.S., F.L.S. 



Read February l/th, 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 occupied the dorsal region, and the branchial network exhibited 

 thi'ee or four principal transverse bars, intersected at right angles by the more numerous 

 longitudinal nervures. The orifice of entry was guarded by a circlet 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. 



A small spur-like appendage or caudex was sometimes distinctly observed, projecting 

 from the dorsal surface of the pedicles, just below the branchial chamber. 



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 ova, 

 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 a dull amber-coloured or reddish yelk. 



The process of yelk-cleavage was easily traceable in a selected scries of ova ; and where 

 that of differentiation had commenced, the ^iteUine mass appeared to be encircled almost 

 completely by a long and gradually tapering tail, wlnle three short sucker-tubes diverged 

 from an opposite point. In more advanced examples, the transparent polygonal cells, in- 

 cluding the true embryonic structures, formed an oval tadpole-Uke body, from the delicate 



