W. K. BROOKS ON THE GENUS SALPA. 35 



afterwards formed. They are not blastodermic but follicular, and they 

 therefore do not repeat the ancestral history in all details, but their 

 changes of position are quite intelligible on the assumption that they are 

 a record which has been preserved from a time when the spiracles them- 

 selves moved up to the middle line of the back and fused to form the 

 cloacal aperture. 



The changes which take place in the position of the aperture during 

 the growth of the embryo are most interesting. The first trace of it, 

 Plate XLI, Fig. 2, g v , is at the upper end of the long vertical axis of the 

 embryo, and the space between it and the ganglion, s, is about equal to 

 the space between the ganglion and the mouth, z ; and the axis of the 

 mouth and that of the cloacal aperture make an angle of about 90. 



As the embryo grows, Figs. 3 and 5, the mouth and the ganglion 

 preserve essentially their original relations to each other, but the space 

 between the ganglion and the cloacal aperture gradually increases until, 

 at last, mouth and cloacal aperture come, in the adult Salpa pinnata, to 

 lie in the same axis at opposite ends of the body, as is shown in Plate I, 

 Fig. 1. 



In the fixed ascidians the mouth and the cloacal aperture are close 

 together, with the ganglion between them, and in this, as well as in other 

 respects, the young salpa embryo is much more like a fixed ascidian than 

 the adult, and I think we must see, in the primary position of the cloacal 

 aperture, evidence that the salpae are descended from fixed ascidians, or, 

 at least, from ancestors very similar to the ascidians in structure and 

 habits. 



We have now traced the broad outlines of the history of the digestive 

 organs, and of the perithoracic tubes and their derivatives, and we will 

 pass to other systems of organs. 



Returning to the stage shown in Plate X, Fig. 8, we have seen that the 

 outer wall consists at this time of a somatic layer of follicle cells, 7, which 

 is continuous, over the area 10, with the central mass of follicle cells, 

 8, and blastomeres, 9. The area, 10, where the two layers are continuous, 

 marks what is to be the middle line of the dorsal surface of the embryo, 

 and some of the blastomeres soon move outwards along this line until 

 they pass entirely out of the follicle and lie directly under the epithelial 

 capsule. These cells are the ectodermal blastomeres, and they are shown 

 at a" in cuts A and B and in Plate XXII, and at 9' in Plate XVII, Fig. 

 5, and in Plate XII, Figs. 1 and 2. The epithelial capsule, which at first 

 passes over them without interruption, as shown in cut A, soon becomes 



