206 PROFESSOR HUXLEY ON THE ANATOMY 



the outer and tlie inner tunics, except at tlie 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 atrkd 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 tlie 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 alimentary 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 Avith the two spaces, or lateral atria, which lie between the 

 branchial sac and the body-wall, and are Avell 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 Avhat has been said that the Avail of the branchial sac of Pijrosoma (and, 

 I may add, of all Ascidians Avith 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 Avords, 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, hoAvever, 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 haemal 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 *. 



* lu the living condition, as Milne-Edwards has hinted, and as I have shown in my memoir {I. c. p. 583), the cilia 

 upon opposite sides of a branchial stigma move in opposite directions. 



