W. K. BEOOKS ON THE GENUS SALPA. 295 



middle line to form the atrium is shown at D in his Fig. 25, and at D and 

 Dh in his Fig. 28, A. 



In the series of sections in his Plate 13, Fig. 31, A shows the two 

 perithoracic tubes cut above the level of the atrium as in my Plate XIII, 

 Fig. 6, and in the diagram on page 30. His Figs. 31 B and 31 C show 

 the atrium as in my Plate XIII, Fig. 7, and his Fig. 31, D shows the two 

 tubes below the level of the atrium but above the level of the pharynx, 

 as in my Plate XIII, Fig. 8. 



Salensky regards these structures as the halves of the primitive 

 digestive cavity which, he says, p. 114, arises in Sal pa 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 repre- 

 sented in his Plate XIV, Fig. 37, and Plate 15, Fig. 39. 



In his Plate 24, Fig. 1 appears to be a section through one of the 

 perithoracic tubes, Pmdh, before it has lost its connection with the sur- 

 face, and in his description of this figure (5), p. 347, he says that the 

 triangular primitive digestive cavity is united above to the epithelial 

 capsule (Ectodermkeim), and on both sides of the tip are the reflections 

 of the somatic layer (follicle cells) already noted, where this passes on 

 into the visceral (gonoblastic) layer. 



I have shown already, in the chapter on the follicle, p. 258, that 

 Salensky correctly describes the manner in which the perithoracic struc- 

 tures (primitive digestive cavity) acquire their epithelial lining by the 

 invagination of the somatic layer (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. 



SECTION 9. The History of the Perithoracic Structures of the 



Aggregated Salpcc. 



I have nothing to add to my account of the origin of the perithoracic 

 tubes of the stolon on p. 75, or to my account on p. 79 of the way in 

 which they are cut up into perithoracic vesicles by the folds of ectoderm. 

 There are two of these vesicles in each salpa, and they give rise to the 

 perithoracic system and to nothing else. Throughout its whole history 

 the perithoracic system is bilaterally symmetrical, although this sym- 

 metry is hidden by the changes which take place in the position of the 

 plane of symmetry during the growth of the embryo. 



