AND DEVELOPMENT OF PYROSOMA. 215 



and the sacs tliemselves are both 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 wliich 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 reot 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 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 stig- 

 mata, 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 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 distuict 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 ; bixt 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 

 boimdaries 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 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 liecome 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 ajiplied 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. 



