132 Mr. W. K. Brooks on the 
embryo is obviously the one which is marked A? in the older 
one. 
The perithoracic tubes are actually shown in many of his 
figures, notably in his plate vi. fig. 5, pin, where they are 
marked Drm. They are also shown in his plate xii. fig. 24, 
at Dh, and their union on the middle line to form the cloaca 
is shown at D in his fig. 25 and at D and Dd in his fig. 28 a. 
Jn the series of sections on his pl. xii., fig. 31.4 shows the 
two perithoracic tubes cut above the level ot the atrium. His 
figs. 81 B and 31 show the atrium, and his fig. 31 D shows 
the two tubes below the level of the atrium but above the 
level of the pharynx. 
Salensky regards these structures as the halves of the 
primitive digestive cavity, which, he says (p. 114), arises in 
Salpa pinnata as two independent and completely separated 
halves ; and he describes the atrium and gill as arising at a 
very much later stage in the way which is represented in his 
plate xiv. fig. 37, and plate xv. fig. 39. 
In his plate xxiv., fig. 1 appears to be a section through 
one of the perithoracic tubes (Pmdh) before it has lost its 
connexion with the surface, and in his description of this 
figure (p. 846) he says that the triangular primitive digestive 
cavity is united above to the epithelial capsule (“ Ectoderm- 
keim’’), and on both sides of the tip are the reflections of the 
somatic layer of the follicle (follicular wall) already noted, 
where this passes over into the visceral (gonoblastic) layer. 
Salensky correctly describes the manner in which the peri- 
thoracic structures (primitive digestive cavity) acquire their 
first epithelial lining by the migration of the somatic layer of 
the follicle (follicular wall), and I believe that I have now 
carried the analysis of his observations far enough to prove 
that they contain internal evidence of the correctness of my 
own account. 
The History of the Perithoracic Structures of the Aggre- 
gated Salpa.—The rudiment of each chain Salpa contains 
two perithoracic vesicles, a right one and a left one, derived 
from the right and left perithoracic tubes of the stolon. 
These vesicles give rise to the perithoracic system and to 
nothing else. ‘Throughout its whole history the perithoracic 
system is bilaterally symmetrical, although this symmetry is 
hidden by the changes which take place in the position of the 
plane of symmetry during growth. 
As the right and left pharyngeal pouches are formed the 
perithoracic vesicles are folded inwards by the growth of the 
ectodermal folds of the stolon, so that each one of them les 
on the proximal or dorsal surface of its corresponding pha- 
