THE DEVELOPMENT OF AMPHIOXUS 



47 



the burrowing habit, serving to protect the delicate and com- 



plicated pharynx. The enclosure of the branchial region by the 



atrium occurs early during the larval period. About the time 



six or seven primary gill slits 



are present, a pair of low folds 



appears in the middle of the 



body region, along the ventral 



surface. These are the so- 



called metapleural folds, at first 



merely thickenings of the ecto- 



derm, but later hollow ridges 



of considerable size; their pri- 



mary cavities are to be re- 



garded as ccelomic in origin. 



The metapleural folds lie close 



together and gradually extend 



forward, the right fold proceed- 



ing rapidly in advance of the 



left and passing far up on the 



right side of the pharynx, 



above the primary gill slits, 



which it reaches about the 



time ten are formed (Fig. 17, 



A ) . Later the left fold reaches 



the gill-slit region, diverges 



widely from the right fold, and 



, . , FIG. 17. Diagrams of three larvae of 



passes tO the Opposite Side Ot Amphioxus, viewed ventrally, showing 



the secondary gill slits, lying 



at first in the topographical Lankester and Willey. A. Folds still 



i / i t i completely separate. B. Folds fused, 



ventral region (morphological and atria i cavity thereby established, 



left Side) (FigS. 14, B; 17). from atriopore to posterior end of 



pharynx. C. Folds fused throughout, 



The metapleural folds enlarge except in the region of the first two 



and new cavities of lymphatic 



character appear within them, left metapleural fold; m, mouth; rm, 

 , . , . , right metapleural fold. 



which encroach upon the 



original coelomic space and lead to their obliteration. 



From the inner face of each metapleural fold a horizontal 



