206 PROFESSOR HUXLEY ON THE ANATOMY 
the outer and the inner tunics, except at the atrial orifice, where it passes into the former, 
and at the anus, where it becomes continuous with the latter. 
Except for these two breaks, the membrane in question (which is the third tunic of 
Milne-Edwards, and is what I have elsewhere termed the atrial tunic) might be compared 
to a closed "serous sac, reflected over the viscera, on the one hand, and over a part of the 
external tunic, on the other, but leaving a space between itself and both these parts, which 
space.is filled with blood, and forms a part of the general system of sinuses of the body, 
A careful examination of the side view (PL XXX. fig. 1), the upper view (fig. 3), and the 
transverse section (fig. 2) will render this statement intelligible. 
In the first, the atrial tunic is seen to be reflected over the posterior face of the stomach 
and first part of the intestine, and then to form the roof and the floor of the cavity, which 
lies between the intestine and the atrial aperture, and which I shall term the mid- 
atrium. In fig. 3, the atrial tunic is shown to be continued forwards at the sides of the 
intestinal canal on to the pharynx, united with which, it forms the branchial sac. 
Arrived close behind the peripharyngeal ridge, it is reflected on to the external tunic, 
and then passes directly backwards to the atrial aperture. The testis and ovisac, which 
are seen, in this view, over the alimentarv canal, lie altogether above the roof of the 
mid-atrium (fig. 1), and therefore do not in any way interfere with the free and wide com- 
munication of the mid-atrium with the two spaces, or lateral atria, which lie between the 
branchial sac and the body- wall , and are well shown in the transverse and vertical section 
(fig. 2). Both in this section and in fig. 3, short cords are seen to pass between the pari- 
etal and the visceral layers of the lateral atria. They are hollow, and place the parietal 
sinuses in communication with those of the branchial sac. 
It follows from what has been said that the wall of the branchial sac of Pyrosoma (and, 
I may add, of all Ascidians with a similar respiratory apparatus) consists, internally, of 
that portion of the alimentary tract which lies in front of the oesophagus and behind the 
mouth (or, in other words, of the pharynx), and, externally, of the visceral layer of the 
atrial tunic. Now, these two membranes do not remain entirely separated by the inter- 
posed sinus, but are united at regular intervals, so as to give rise to hollow vertical bars 
separated by equally long vertical clefts— the branchial stigmata. 
Of these stigmata there are about thirty on each side. The most anterior and the most 
posterior ones are shorter than the others. Anteriorly, in fact, the first is not more than 
one-third or one-fourth, or even less, as long as the vertical height of the branchial sac. 
The stigmata, however, increase in length up to the sixth, and then acquire nearly the 
height of the sac, so as to leave only a small imperforate space on each side of the lan- 
guets, on the neural side, and of the endostyle, on the hamial side. Posteriorly the last 
four or five also gradually diminish, until the hindmost of all is not larger than the 
foremost . 
The vertical bars bounding the stigmata are fringed by a single series of elongated cor- 
puscles, each of which bears a row of long cilia, and (in the dead state, at any rate) all 
these cilia project outwards into the lateral atria *. 
condition, as Milne-Edwards has hinted, and as I have shown in mv memoir (7. c. p. 583), the ci ia 
'e 
upon opposite sides of a branchial stigma move in opposite directions. 
