W. K. BROOKS ON THE GENUS SALPA. 



107 



SECTION 6. The Rotation of the Salpce. 



The rotation of the body upon its own axis is a simple process, and 

 the only perplexing complication comes from the fact that the whole 

 body does not rotate at the same time. The 

 rotation begins at the aboral end of the body, 

 and gradually extends along the axis towards 

 the oral end, which rotates last. The way in 

 which it takes place is well shown in cut Y, 

 which is a series of salpae of Salpa africana, 

 which will also serve to illustrate the simplest 

 method of forming two series from a single row 

 of salpae, as the process in this species is essen- 

 tially that which is represented in diagram W. 



This figure is a horizontal longitudinal sec- 

 tion through a stolon of Salpa africana at the 

 stage in which the salpae have pushed to the 

 sides and have formed two rows. The lower 

 or aboral ends of their bodies have rotated so 

 that their dorsal surfaces are external, while 

 the upper or oral ends are more nearly in their 

 primitive positions. The salpae are numbered, 

 in their morphological order, from 1, at the dis- 

 tal end, to 21, at the proximal end, and sections 

 1, 2 and 3 are shown, more enlarged, in Plate 

 XXXVII, Fig. 26; sections 4 and 5 in Fig. 25; 

 and 6 and 7 in Plate XLVI, Fig. 2. It will be seen that, at the oral end 

 of the body, the ganglia, s, are proximal or towards the base of the 

 stolon, and their right and left sides on its right and left. 



(As the series to which these sections belong was begun at the aboral 

 end, the salpae are placed with their aboral ends towards the observer, 

 and the right side of the stolon, R, is therefore on the left of the figure.) 



As the stolon of Salpa africana is not straight, but curved in the 

 arc of a circle, as is shown in Plate IV, Fig. 2, a longitudinal section of 

 the stolon will cut successive salpae at different levels, so that a single 

 section, like the one which is figured, not only shows the relation of the 

 bodies to each other, but also gives what is equivalent to a consecutive 

 series of sections through a single salpa, although the curvature of the 

 stolon is such that no two of them are in strictly parallel planes, and the 



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