244 University of California Publications in Zoology [Vol. 18 



order to illustrate these secondary changes in position let us represent the 

 series of salpae 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 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 toward the backs of those in the other 

 row. They will now illustrate two rows of salpae. 



To make the illustration more perfect suppose that, instead of stepping 

 into their new places, the soldiers grow and 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 twist 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 

 develops from a simple germ to his complete structure. 



In Salpa democratica the stolon undergoes more or less regular 

 periods of active segmentation and rest so that the aggregate salpae 

 are developed in sets or blocks, all individuals in a single block being 

 of approximately the same size and in the same stage of development. 

 Leuckart (1854, p. 67) found forty in one block and sixty-five in 

 another; while Seeliger (1886, p. 593) counted sixty-one in a single 

 block. As there are from three to four blocks present when the distal 

 end of the chain is ready to emerge from the mantle cavity to the 

 exterior, the stolon carries in the neighborhood of two hundred salpae 

 at one time. No evidence is at hand that the stolon ever exhausts 

 its capacity for producing them, and segmentation probably continues 

 until terminated by the death of the solitary salpa. 



Again, in 8. democratica, a later position is assumed by the salpae 

 such that those in one row of the chain alternate with those in the 

 other row, each salpa being connected by two processes from its body 

 wall with the one ahead of it, by two with the one behind it, and by 

 two to each of the two adjacent ones diagonally opposite it in the 

 other row. To quote again from Brooks (1893, p. 88) such a chain 

 of salpae "may be compared to two trains of cars on two parallel 

 tracks, placed so that the middle of each car on one track is opposite 

 the ends of the two, cars on the other track, and each joined by two 

 couplings to the car in front of it on its own track, and in the same 

 way to the one behind it, and also to those diagonally in front of it 

 and behind it on the other track." In this position "the long axis 

 of the salpae are at right angles to the long axis of the stolon, as if the 

 cars in the two trains were set on end." Now imagine the cars in 

 each train to be pushed over until each one rests upon the one in 

 front of it, and at the same time imagine each car to rotate so that 

 it becomes inclined outward at an angle of about forty-five degrees. 



