80 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



order to illustrate these secondary changes of position let us represent 

 the series of salpse 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 towards the backs 

 of those in the other row. They will now represent two rows of salpae 

 like those shown in Plate IV, Fig. 1. 



To make the illustration more perfect suppose that, instead of step- 

 ping into their 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 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 also develops from a simple 

 germ to his complete structure. 



As thus outlined these secondary changes are simple, but as growth, 

 development, pushing to the side, and rotation are all going on together, 

 and as they all take place gradually, the interpretation of sections is 

 very puzzling ; although the history of their development becomes very 

 simple when it is illustrated by diagrams from which the secondary 

 changes are omitted, instead of by figures from actual sections. 



V. The Development of the Chain-Salpa. 



(1) The body cavity. At first this is directly continuous, through the 

 blood spaces i andj, with those of the adjacent salpae, but it is gradually 

 shut in by the growth of the ectodermal folds, of which the inner edges 

 is shown in the diagrams and in the figures on Plate V. 



(2) The pharynx. The lateral pouches, Plate V, Fig. 1, 27 and 28, 

 from the sides of the endodermal tube, d', grow forwards towards the oral 

 end of the body, cut N, 27 and 28, o, and backwards towards the aboral 

 end, cut 0, 27 and 28. They are the rudiments of the right and left 

 halves of the branchial sac or pharynx, and they may therefore be called 

 the pharyngeal pouches. 



Their oral ends soon bend in towards each other, cut 0, and finally 

 meet and unite on the middle line, cut P, ventral to the ganglion, s, to 

 form the oral end of the pharynx, which is shown at 28-O-27 in cuts P 

 to X. Finally, the oral end of the pharynx unites with the ectoderm 

 and becomes perforated to form the mouth, cut S, z. 



