W. K. BEOOKS ON THE GENUS SALPA. 75 



It is difficult to determine whether this endothelium is formed in 

 place, as is probably the case, or derived from the endothelium of the 

 blood spaces of the embryo, for its great delicacy in young stolons renders 

 it unfavorable for study in sections. A fragment of it is shown in Plate 

 XXI, Fig. 6, above the letter g, and it is also shown in Figs. 2, 3, 4 and 5. 



SECTION 7. The Perithoracic Tiibes. 



These are colored green in the plates, and the right one is marked gr, 

 and the left one h. 



They run along each side of the stolon, between the ectoderm and 

 the thickened side walls of the endodermal tube. In mature stolons, each 

 of them has a distinct tubular lumen, as is shown in transverse section 

 in Plate XXXIV, Fig. 3, h, and in longitudinal section in Fig. 6, h. In 

 very young stolons, Plate XXI, Fig. 7, g and h, the lumen is absent. 



I have devoted especial attention to the question of the origin of 

 these tubes, but I have not been able to obtain any evidence which I 

 regard as conclusive, although my observations indicate that the tubes 

 arise in the ectoderm of the young stolon, at the points where this joins, 

 at the sides of the stolon, the ordinary ectoderm of the embryo. In old 

 stolons, like the one shown in Plate XXXIV, Fig. 1, the tubes can be 

 traced to the proximal end of the stolon, and at the point where the ecto- 

 derm bends outwards to join that of the solitary salpa they come to an 

 end, although I have not been able to find, in old stolons, any union 

 between them and the ectoderm. The stolon shown in the transverse 

 sections in Plate XX had no perithoracic tubes, although, on the left side 

 of Fig. 3, the fold where the ectoderm, a', of the stolon joins that of 

 the embryo, a, runs down for some distance into the stolon, alongside 

 the endoderm. 



This may possibly be the rudiment of a perithoracic tube, the right 

 one, as these sections were drawn from inverted specimens, but I have 

 not been able to prove to my own satisfaction that it is. 



In Plate XXI, Fig. 2, the left perithoracic tube is shown to lie in very 

 intimate relation to the ectoderm at the root of the stolon close to the 

 point where, as section 1 shows, the ectoderm folds upon itself. This 

 section, like the one before it, seems to show that the perithoracic tubes 

 arise from the ectodermal fold at the proximal end of the very young 

 stolon, and I know of no theoretical ground for doubting this evidence, 

 although none of my sections show any vegetative activity in the cells 



