342 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
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 intestine 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 antero-lateral boundary of the 
great posterior sinus. The space between the wall of the pharynx 
and the inner layer of the sac communicates 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 opaque rings with clearer centres, the rudiments of 
the branchial stigmata, make their appearance (figs. 19 and 20). These 
correspond with the points of union 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 repre- 
sented 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 (fig. 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 
