REPOET ON THE THNICATA. 
253 
sac and the intestine and the other viscera are in the form of a mass of granular debris 
mixed vuth mud, &c., evidently food matters from the alimentary canal. 
The British Museum specimen, which, through the kindness of Dr. Gunther, I was 
permitted to examine, is in better histological condition, and shows the following points 
in the structure of the Ascidiozooids. The mantle is well developed and is fairly muscular. 
The chief muscle bands run longitudmally. The branchial sphincter is strong. The 
endostyle is short but broad, and has an undulating com’se. The tentacles are all of one 
size and are numerous. The nerve ganglion is nearly spherical in form. 
None of the Ascidiozooids examined showed either the branchial sac or the alimentary 
and reproductive viscera in sufficiently good condition to allow the details of their 
structure to be determined. A large conspicuous vas deferens is, however, present, and 
as it extends for a considerable distance behind the thorax, it may be inferred that a 
post-abdomen was present. 
It is more from the general appearance of the colony and of the Ascidiozooids than 
from any special points in the anatomy that I place this form in the Polyclinidse, and in 
the absence of further information in regard to its structure it is impossible to refer it 
to its proper genus. 
I am inclined to think that both the Challenger specimens and the British Museum 
specimen were dead and decomposing colonies when they were found and put in spirit. 
Giard has shown that in some species of Compound Ascidians it is customary at 
certain seasons for the entire colony to die, and I have myself observed, both 
on the west coast of Scotland and also in the Chausey Archipelago, off the coast 
of Brittany, many colonies belonging to several species of Polyclinidae in a dead and 
decaying condition. The test is usually in these cases in a soft and spongy state, 
with an irregular outer surface, and the Ascidiozooids are many of them partially 
or completely expelled from the colony, just as is the case in the Challenger specimens 
under consideration. It is to be hoped that some future explorers in the Southern 
Seas may be successful in obtaining specimens of this, probably the largest known, 
species of Compound Ascidian in a living condition. 
Family IV. Didemnid^. 
Colony usually flat, thin, and incrusting, rarely thick and massive, never 
pedunculated. 
Systems complicated and irregular, inconspicuous, or absent. Common cloacal 
apertures usually conspicuous. 
Ascidiozooids rather small, divided into two regions — thorax and abdomeu. 
Branchial aperture six-lobed, atrial plain, or provided with a languet. 
Test gelatinous or cartilaginous, usually containing numerous stellate calcareous 
