104 JOHNS HOPKINS UNIVERSITY MOEPHOLOGICAL MONOGEAPHS. 



The stage which is shown in diagram N may be made the starting- 

 point for our study of the secondary changes, although they begin in the 

 actual salpa before this stage is reached. We must picture to ourselves a 

 stolon made up of a series of segments like the diagram, placed in single 

 row so that only one of them is visible when the series is viewed from 

 either end. The digestive tract of each segment communicates with 

 those of the preceding and following ones through the opening d', which 

 thus forms a continuous endodermal tube, running from one end of the 

 stolon to the other, dilated in each salpa into the two pharyngeal pouches 

 27 and 28, and constricted, where each salpa joins the next one, to the 

 aperture d'. As is shown in Plates XXIII and XXIV, the body cavity 

 of each salpa is partially shut off by the fold of ectoderm, the inner 

 edge of which is shown in the diagram at i and j; while inside this 

 line the body cavity of each one communicates with those of adjacent 

 salpae through the blood spaces which are shown in the diagram, and 

 also in Plates XXIII and XXIV at i and j. 



These blood spaces therefore form continuous tubes like the endo- 

 dermal tube d', running from one end of the stolon to the other. 



Finally we must note that, since the whole stolon becomes converted 

 into salpaB which are directly connected with each other, as Plate XXIV 

 shows, without the intervention of undifferentiated connecting tubes, it 

 follows that the endodermal tube is, in every section, part of the pharynx 

 of some salpa, and that every part of the blood-tubes is, in the same way, 

 part of the body cavity of a salpa. 



Now imagine that these rudimentary, ring-like salp move alter- 

 nately to the right and left, in the directions shown by the arrows, so 

 that they reach the places which they occupy in Plate VIII, Figs. 1 and 

 2, without breaking loose from their connection with each other in the 

 series. In describing this change it will be best to resolve the arrow into 

 a horizontal and a vertical component, and to picture first the movement 

 outwards, which is shown in diagram W, and then the movement down- 

 wards, which is shown in diagram X. 



In Salpa africana (and also in Salpa democratica, Salpa cordiformis, 

 and in most salpaB with curled stolons) the animals move almost 

 directly outwards, in the direction which is indicated by the horizontal 

 arrow ; in Salpa cylindrica they move outwards and downwards in the 

 direction indicated by the oblique arrow, until they assume the positions 

 shown in Plate VIII, Fig. 2, and finally, in Salpa pinnata, Plate VIII, Fig. 

 1, the line of movement is rather downwards than outwards in the direc- 

 tion indicated by the third arrow. 



