88 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



These figures show that the process is ectodermal, hollow, and that 

 the ectoderm of adjacent salpae comes into contact at the end of the pro- 

 cess. After the cellulose mantle is formed the processes come to consist 

 almost entirely of cellulose, although the actual contact between adjacent 

 salpae is ectodermal. Plate XXXVII, Fig. 21, shows that in Salpa pinnata 

 each individual is united by it to four others, the two which are diago- 

 nally opposite it on the other side of the series, and the two adjacent to 

 it on its own side. As the wheel-shaped colonies, Plate I, Fig. 2, are 

 formed, the processes from all the members meet in the center and bind 

 them together. 



In all the other salpae, each individual in the chain is joined on to 

 four others, the adjacent ones on its own side of the chain, and the alter- 

 nating ones on the opposite side, but instead of being effected by a single 

 process as it is in Salpa pinnata, the union is usually brought about by 

 eight, as is shown in the figure of Salpa scutigera, Plate IV, Fig. 1, and 

 Salpa cordiformis, Plate IV, Fig. 6. We know of no species which stand 

 midway between those of the pinnata group and the ordinary salpae, and 

 we therefore have no phylogenetic evidence, but it seems probable that 

 Salpa pinnata gives us the primitive method, and that originally a single 

 process joined each salpa on to four others, and that this single process 

 has been gradually converted into eight separate ones. In all cases the 

 processes are primarily ectodermal, and they are shown at an early stage 

 in Salpa cylindrica, in section in Plate XXXVII, Fig. 26. 



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 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 young chains, of all species, on the stolon, the long axes 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, and this primitive position is, as I have 

 said, persistent in Salpa scutigera and Salpa bicaudata. 



Salpa democratica and Salpa tilesii pass through this stage, and 

 before their chains are set free the bodies of all the individuals become 

 inclined in the same direction, as, if the cars in the train were pushed 

 over till each one rests against the one in front of it, Plate XLIII, Fig. 1. 



This change takes place in such a way that the oral end of the body 

 of each salpa is thrown towards the distal end of the stolon, and in the 



