ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA = 343 
stomach by a comparatively narrow and short canal (f), which is the 
mid-atrium. 
I have not traced out all the details of the process of coalescence 
of the lateral atria; but I suppose that each branchio-parietal portion 
of the atrium, at first a distinct sac, is prolonged downwards and in- 
wards, under the stomach, and that the opposed walls of the prolonga- 
tion become applied to one another, coalesce, and then become 
perforated. At any rate, the mid-atrium is now surrounded by a 
membranous wall, continuous on all sides with the lining of the lateral 
atria, and applied superiorly and anteriorly against the stomach and 
cesophagus, posteriorly and inferiorly against the external tunic, but 
not touching either of these parts, except for a small space on the 
floor of its chamber, where it becomes united with the external tunic 
to allow of the formation of the atrial aperture. In the present bud 
(fig. 21) this aperture is situated on the neural side of the body, in 
front of the posterior end, which is chiefly occupied by the genitalia ; 
but as development goes on, the mid-atrium increases disproportion- 
ately, and encroaches upon the other organs, upwards and forwards, 
in such a manner that its anterior wall invests the whole posterior 
and lateral faces of the gastro-intestinal division of the alimentary 
canal; while its roof (to speak metaphorically) thrusts the genitalia 
altogether into the hemal region of the body, and its posterior and 
inferior walls, extending backwards, carry the external tunic with 
them, and eventually cause the atrial aperture to take its place at that 
extremity of the body which is directly opposed to the mouth, and 
far behind the genitalia (see figs. 22-25). 
The communicating apertures between the mid-atrium and the 
lateral atria increase in size par¢ passu with the growth of the parts ; 
and hence, in the fully formed ascidiozooid, the gastro-intestinal 
division of the alimentary canal is enclosed in a sort of vertical 
mesentery (formed by the anterior wall of the mid-atrium in the 
middle line, and the internal wall of the lateral atria at the sides), 
whose layers are continued, on either hand, into the outer wall of the 
branchial sac. At the anterior boundary of the branchial sac they 
are reflected into the outer or parietal layer of the lateral atrium. 
The facts which I have detailed! are exceedingly important for 
the comprehension of Ascidian structure in general. From its mode 
of development, it is perfectly obvious that the inner wall of the 
branchial sac of Pyrosoma is not composed of tentacles which have 
1 The accurate Krohn, in his account of the development of Phad/uséa (Miiller’s ‘“ Archiv,” 
1852), was the first to note the separate origin and subsequent confluence of the lateral atria. 
In this genus, however, each lateral atrium has, at first, a distinct external aperture. 
