Origin of the Organs of Salpa. 125 
changes, their accounts of the origin of the aggregated Salpx 
have no permanent value. 
I pointed out in 1886 (‘ Studies from the Biol. Lab., Jolins 
Hopkins Univ.,’ 1886, pp. 898-414) that the Salpa chain is 
morphologically a single row of Salpz, all in the same 
position, with their dorsal surfaces proximai or towards the 
base of the stolon and their right sides on its right. The 
account of the origin of the aggregated Salpz 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 bilateraliy symmetrical, its plane of symmetry 
is fundamentally identical with that of the solitary Salpex, 
and the rudiment 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 
Salpe and a series of left-hand ones placed with their dorsal 
surtaces 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 
Salpz by a file of soldiers, all facing the same way. Now 
imagine that each alternate soldier moves to the right and 
the others to the feft, 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 Salpz 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 
Salpez; 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. Ilist. Ser, 6. Vol. xii. 10 
