W. K. BKOOKS ON THE GENUS SALPA. 113 



axis of the body, as shown also in the figures on Plates XXXIX and XL. 

 (Through an oversight in the lettering of Fig. 15 of Plate XL, the letter 

 d l which is elsewhere used for the right half of the endostyle is used 

 here for the left, and d, 2 for the right.) 



Soon after the stage which is shown in these figures, the axial por- 

 tion of the body of Salpa cylindrica becomes entirely shut off from the 

 lateral portion, which becomes converted into a complete salpa, while 

 the axial portion which is morphologically a section taken out of the 

 middle region of its body degenerates and disappears, the endodermal 

 portion being the first to lose its connection with the salpa proper. 



In Salpa pinnata these changes take place at a very much younger 

 stage, and for this reason they are difficult to follow without the guid- 

 ance of Salpa cylindrica. 



SECTION 7. The Development of the Aggregated Form of Salpa pinnata. 



The direction of the movement outwards by which the single row of 

 salpae becomes two rows differs in different species. In Salpa africana 

 the movement is almost directly outwards, and Seeliger's figures show 

 that this is true of Salpa democratica also. In Salpa cylindrica the 

 movement is outwards and downwards in the direction indicated by 

 the arrows in cut N, as is also shown by the figure of Salpa cylin- 

 drica in Plate VIII, Fig. 2. In Salpa pinnata the movement is more 

 downwards than outwards, as the figures in Plates V, VI and VII 

 show. So far as the secondary changes are concerned, Salpa africana 

 and Salpa democratica are the simplest, and they would be the easiest to 

 understand if the stolon were straight. Salpa cylindrica comes next in 

 intelligibility, but as its stolon is straight it is actually a more favorable 

 subject for study. In Salpa pinnata the secondary changes are most 

 obscure and difficult to trace, but the fact that the series of animals in 

 its straight stolon develop in succession instead of in sets, renders it in 

 many respects the most favorable species to study, and as its secondary 

 changes are fundamentally like those of the other species, they should 

 present no great difficulty after the history of Salpa africana and Salpa 

 cylindrica is understood. 



I have selected this species, Salpa pinnata, for the most extended 

 illustration, since, on the whole, it is the most instructive species, and 

 the advantages it affords by its straight stolon and by the gradual 

 development of the salpse more than compensate for the obscurity which 

 comes from the very early stage at which the secondary changes occur. 



