W. K. BROOKS ON THE GENUS SALPA. 291 



SECTION 7. The Ganglia of the Aggregated Salpce. 



The nerve tube arises as a solid rod, Plate XX, Fig. 4, I, but it soon 

 acquires a lumen, Plate XXI, Fig. 7, /. As the ectodermal folds grow 

 inwards and mark out the bodies of the salpae, they cut the tube up, 

 Plate XV, Fig. 10, s, into a series of ganglionic vesicles, one for each 

 salpa, with cavities which are segments of the lumen of the tube. 



The presence of the ectodermal folds and the growth of the ganglion 

 soon cause crowding and pressure, and the ganglia, Plate XV, Fig. 10, s, 

 and Plate V, Fig. 1, s, become flattened in the axis of the stolon and 

 elongated transversely. As the oral ends of the pharyngeal pouches, 

 Plate XXIV, Fig. 5, 27 and 28, grow up to the level of the ganglion, and 

 push in between it and the ectoderm, as shown in cut O on page 81, the 

 crowding becomes still greater, and the single series of ganglia breaks 

 up into two rows, which move to the right and the left alternately, as 

 shown at s in Plate XXXI, Figs. 1, 2 and 3, B and O. 



I have already shown that when the salpae are in their primary or 

 morphological positions the dorsal surface of each is turned towards the 

 root or proximal end of the stolon and its ventral surface towards its 

 tip. In the series of figures on Plate XV and Plates XXIII and XL, 

 all the sections are figured with the proximal end of the stolon at the 

 bottom, so that, so long as the primitive position is retained, the dorsal 

 surface of each section is below. Most of the reconstructions of the 

 transverse sections on Plates V to VIII are proximal views like the 

 diagrams on pp. 81 and 82. 



As the pharyngeal pouches push in behind the ganglion they meet 

 and unite on its ventral or distal side, Plate XXXI, Fig. 3, B-B, so that, 

 from this time on, the ganglion occupies its normal position above the 

 dorsal middle line of the oral end of the pharynx, and its changes of 

 position with reference to the axis of the stolon, which are shown at S 

 in Plate V, Figs. 1, 2, 3, 4 ; Plate VI, Figs. 1, 2, 4 ; Plate VII, Figs. 3, 4, 5, 

 and Plate VIII, Fig. 1, are due to secondary changes in the position of 

 the middle plane of the body of the salpa as already explained, page 109, 

 and not to any change in the relation of the ganglion to the other organs 

 of the body. 



It is convenient, however, to describe the ganglion as "migrating" 

 outwards from the middle line to the right in a right-hand salpa, Plate 

 V, Figs. 1, 2 and 3 ; and to the left in a left-hand one, Fig. 4, and moving 

 downwards, Plates V, VII and VIII, until, in Salpa pinnata, it comes 



