W. K. BROOKS ON THE GENUS SALPA. 67 



As this series of figures shows, the stolon of Salpa pinnata lies below 

 the middle line of the ventral surface of the solitary salpa, with its free 

 end pointing forward. In several of the figures, as in Plate XVI, Fig. 4, 

 Plate XXI, and Plate XLV, Fig. 5, the two folds d-d which compose the 

 endostyle of the solitary salpa are shown, and it will be seen that the 

 stolon lies under the furrow in the middle line of the ventral surface of 

 the pharynx between these folds. 



The figures in Plate XXI also show that the vertical longitudinal 

 plane, between the halves of the endostyle, which cuts the solitary salpa 

 into symmetrical halves, also cuts the stolon symmetrically, so that Plate 

 XXXV cuts both the embryonic solitary salpa and the stolon in the 

 plane of bilateral symmetry. When in its true or morphological position, 

 then, the stolon is symmetrical with reference to the middle plane of the 

 solitary salpa, although, as the stolon grows older, it is usually somewhat 

 twisted on its own axis, as Plate XVI, Fig. 4, and Plate XLV, Fig. 5, 

 show. 



In many species of Salpa, the stolon, as it grows, becomes wrapped 

 in a spiral around the digestive organs, as is shown at st, in the figure 

 of the solitary Salpa africana, in Plate IV, Fig. 2, and when this is the 

 case, the stolon begins to curve to one side almost as soon as it makes 

 its appearance. There can be no doubt, however, that in all cases it is, 

 morphologically, a median structure, symmetrical in the same plane as 

 the solitary salpa ; that Salpa pinnata shows its true or primary position, 

 and that its position in species like Salpa africana is secondary. 



When fully formed it consists, in cross section, Plate XXXIV, Fig. 

 1, of the following structures: 1st, a tubular sheath of ectoderm, a; 2d, a 

 nerve tube, /; 3d, an endodermal tube, d'; 4th, two perithoracic tabes, 

 (/ and li ; 5th, two blood-tubes, i and j, and 6th, a genital rod, m-n. In 

 addition to these well-defined constituents, there are isolated cells between 

 the cloacal tube and the adjacent structures, shown, uncolored, in Plate 

 XXXIV, Fig. 3. 



Before the chain-salpae make their appearance, all these structures 

 run from the base of the stolon nearly to the tip, so that every section is 

 like the figure, but at the tip the nerve tube, genital rod and perithoracic 

 tubes disappear, and the endodermal tube d' ends blindly, as the longi- 

 tudinal section, in Plate XVI, Fig. 4, shows, so that the two blood-tubes 

 communicate with each other. They do not communicate with the 

 exterior, however, or with the cavity of the endodermal tube. 



