Origin of the Organs of ^2iX^[\. 12 J 



changes, their accounts of the origin of the aggregated Salpze 

 have no permanent value, 



I pointed out in 1886 {' Studies from the Bioi. Lab., Johns 

 Hopkins Univ,,' 1886, pp. 398-414) that the Salpa chain is 

 morphologically a single row of Salpa?, all iu the same 

 position, with their dorsal surfaces proximal or towards the 

 base of the stolon and their right sides on its right. The 

 account of the origin of the aggregated Salpae which is given 

 in this memoir is simply an amplification and expansion of 

 the statement which in 1886 I made briefly and in outline. 



The stolon is bilaterally symmetrical, its plane of symmetry 

 is fundamentally identical with that of the solitary Salpai, 

 and the rudim.ent of each aggregated Salpa is bilaterally 

 symmetrical in the same plane, although the secondary 

 changes begin very early, and convert the single row into a 

 double row, which comes to consist of a series of right-hand 

 Salpaj and a series of left-hand ones placed with their dorsal 

 surfaces out, their ventral surfaces towards the ventral sur- 

 faces of those in the opposite row, and with the left sides of 

 those on the right and the right sides of those on the left 

 towards the base of the stolon. In order to illustrate these 

 secondary changes of position, let us represent the series of 

 Salpge by a file of soldiers, all facing the same way. Now 

 imagine that each alternate soldier moves to the right aud 

 the others to the left, to form two files, still facing the same 

 way. Now let them face about, so that the backs of those in 

 one row are turned towards the backs of those in the other 

 row. They will now represent two rows of Salpaj in their 

 secondary positions. 



To make the illustration more perfect, suppose that, instead 

 of stepping into new places the soldiers grow until they are 

 pushed out by mutual pressure, and suppose that their heads, 

 growing fastest, form two rows, while their feet still form one 

 row, and suppose furthermore that, as each soldier rotates, 

 his feet turn first, and that the twisting runs slowly up his 

 body to his head, which turns last. We must also imagine 

 that these various changes all go on together, and that while 

 they are taking place each soldier not only grows larger but 

 also develops from a simple germ to his complete structure. 



Salensky regards the stolon as two rows of rudimentary 

 Salpse ; and while Seeliger correctly states that they at first 

 form a single row, he has failed to discover the rotation, and 

 believes that they arise on the stolon in their final positions, 

 and he has therefore failed as completely as Salensky in his 

 efforts to trace the origin of their organs. 



Ann. & Mag. N. Hist. Ser. G. Vol. xii. 10 



