W. K. BROOKS ON THE GENUS SALPA. 103 



SECTION 5. The Secondary Changes in tlie Position of the Aggregated 



Salpce, upon the Stolon. 



We have now to describe in detail the secondary changes in the 

 position of the salpae upon the stolon. 



As I have already pointed out, these are of two sorts. In the first 

 place, the salpa? in the single series push alternately to the right and to 

 the left, and form two rows instead of one ; and in the second place, each 

 salpa rotates on its own axis, so that in all the salpae the dorsal or neural 

 surfaces, which were originally proximal or towards the base of the stolon, 

 come to face outwards, while the left sides of all the right-hand salpae 

 and the right sides of those on the left become turned towards the 

 proximal end or base of the stolon. 



These changes do not take place in succession, but simultaneously, 

 nor do they take place suddenly after the bodies of the salpas are fully 

 developed. On the contrary they begin at a very early stage, and they 

 go on gradually while the salpae themselves are gradually taking shape 

 and undergoing their development and growth. The difficulty in tracing 

 them or describing them is entirely due to this peculiarity, and to the 

 fact that they must be studied in sections. If the successive stages could 

 be dissected out and studied in surface views, the subject would not be 

 obscure or difficult; but it is obvious that sections in a fixed plane, 

 through a series of developing animals which are constantly changing 

 their relative positions, must necessarily be hard to interpret. 



If the salpae first became fully developed while in their primitive or 

 morphological positions, and then after they were fully formed assumed 

 their secondary positions, the case would be as simple as the formation 

 of two files of soldiers from a single file. 



If there were a species in which the salpa? completed their develop- 

 ment before the secondary changes took place, this species would be the 

 easiest one to understand. There is no such species, and in all which I 

 have studied the secondary changes begin very early, but certain species 

 retain very much longer than others, in the structure of the young chain- 

 salpae, traces of their primitive position and a record of the path they 

 have followed in reaching their secondary position. 



Of the three species which I have studied by sections, Salpa africana 

 is the easiest to understand, Salpa cylindrica next, and Salpa pinnata is 

 the most perplexing. It will therefore be best to speak of these three 

 species in this order. 



