W. K. BROOKS ON THE GENUS SALPA. Ill 



Salpa cylindrica, it takes place very early; and in Salpa pinnata they 

 have met on the middle line and united at the stage shown in Plate VI, 

 Fig. 4 and Plate VII, Fig. 2, and in Plate XXXIII, M-M' and N-N>. 



In Salpa cylindrica the aboral ends of the pouches remain distinct 

 from each other until an advanced stage of development is reached, 

 Plate XXXIX, 27 and 28, although they come together on the middle 

 line and are separated only by a thin flat mesentery, which is shown 

 in the sections as a double layer of flattened endoderm cells, running 

 vertically from the gill, o, to the middle line of the ventral surface of the 

 pharynx between the folds of the endostyle. The connection between 

 the axial portion of the body and the lateral portion also persists in 

 Salpa cylindrica very much longer than it does in any other species 

 which I have studied. 



The position of the endostyle is most instructive in Salpa cylindrica. 

 In Salpa pinnata it does not make its appearance until about the stage 

 shown in Plate VIII, Fig. 1, after the pharynx has assumed its adult 

 form and has entirely lost its connection with the endodermal tube, so 

 that this species tells us nothing whatever about its morphological 

 position, but in Salpa cylindrica it makes its appearance at the stage 

 shown in diagram Q as a thickening or ridge in the ventral wall 

 of each pharyngeal pouch, involving both its aboral and oral portions. 

 As these pouches approach each other and unite, the two folds which are 

 to form the endostyle approach and become parallel at their anterior 

 and posterior ends, while they diverge from each other in the middle 

 region of the body and pass around the endodermal tube. 



As they are differentiated at such an early stage in this species, they 

 are of course involved in the secondary changes of position, and they 

 furnish excellent landmarks for studying the character of these changes. 



Thus, in Plates XXXIX and XL if we fix our attention on the right 

 half of the pharynx, 27, and the right half of the endostyle, in Fig. 10, 

 we can trace them through Figs. 11, 12, 14, 15, 16, 17, 18, 19, 20 and 21. 

 This last section is near the oral end of the body, anterior to the ganglion, 

 Figs. 19 and 20, s, and we see that the right half of the pharynx of a 

 right-hand salpa is a continuous structure, as it is represented at 27 in 

 cut W. 



In a section, which has not been drawn, between Fig. 20 and Fig. 21, 

 it gives rise to the small tube which is shown at 27 in Fig. 21. This 

 tube, which is also shown at 27 in Plate VIII, Fig. 2, is the connection 

 between the right half of the pharynx and the right side of the endo- 



