AND DEVELOPMENT OF PYROSOMA. 215 
and the sacs themselves are hoth absolutely and relatively larger. In fig. 29 they are very 
much larger and thinner, and their relations to other organs are especially worthy of 
attention. The outer layer of each is applied to the outer tunic of its side, leaving a small 
interspace, which communicates freely with the great posterior sinus, in which the intes- 
tine and genitalia are disposed, and with the anterior sinuses which lie between the 
pharyngeal wall and the external tunic. This interspace is, in fact, the parietal sinus. 
The internal layer, continuous with the outer anteriorly and posteriorly, but separated 
from it by a wide chamber for the rest of its length, is applied against the wall of the 
pharynx for four-fifths of the extent of the latter, and then coats the lateral portions of 
the gastro-intestinal tract, forming the anterolateral boundary of the great posterior 
sinus. The space between the wall of the pharynx and the inner layer of the sac com- 
municates anteriorly with the anterior sinuses, posteriorly with the posterior sinuses 
and it is interrupted at several points by the union of the pharynx and inner layer with 
one another. It represents the system of branchial sinuses. 
In side views it is not easy to make out the boundaries of the lateral sacs ; but it is most 
important to observe that, as has been already mentioned, in the middle of the lateral face 
of the pharynx, and, therefore, also in the middle of the lateral face of the inner wall of 
the sac, a series of opake rings with clearer centres, the rudiments of the branchial sti 
mata, make their appearance (figs. 19 and 20). These correspond with the points of uni 
-> 
of the pharynx and the inner wall of the sac. They are, at first, small, round, and very 
indistinct, but, by degrees, they elongate in a direction perpendicular to the long axis of 
the pharynx, and their real nature becomes apparent. Hence it is clear that these stigmata 
must eventually open into the lateral sacs, as indeed they may be seen to do in such buds 
as that represented in fig. 30 ; and hence also it follows, that the lateral sacs are the 
rudiments of the lateral atria. 
At first the lateral atria appear to be perfectly distinct from one another, and no atrial 
aperture is discernible. In buds such as that represented in fig. 29, again, they do not 
extend, posteriorly, further than the sides of the alimentary canal ; but in more advanced 
buds (fig. 30) they are produced backwards on each side until they pass beyond the level 
of the posterior margin of the stomach, so that they now constitute the entire lateral 
boundaries of the great posterior sinus. The longitudinal section (tig. 21) of a somewhat 
smaller bud than that represented in fig. 30 shows, however, that, in this condition, the 
atria are no longer distinct, but are united together below the stomach by a comparatively 
narrow and short canal (p), 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 inwards, under the stomach, and that the opposed walls "of the 
prolongation 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 oesophagus, 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. 
