132 Mr. W. K. Brooks on (he 



embryo is obviously the one which is marked Kl 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 Z>?w, 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 I) and Dh in his fig. 28 A. 

 In the series of sections on his pi. xiii., fig. 31 A shows the 

 two perithoracic tubes cut above the level of the atrium. His 

 figs. 31 B and 31 C 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. 346) 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 

 lliat 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 ];haryngeal 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 lies 

 on the proximal or dorsal surface of its corresponding pha- 



