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, 



Later the left fold reaches 

 gill-slit region, diverges 



A). 

 the 

 widely from the right fold, and 



. , FIG. 17. Diagrams of three larvae of 

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



the Secondary gill slits, lying the , relations of the metapleural folds 



' and the method of their closure. After 



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



i / 11-1 completely separate. B. Folds fused. 



ventral region (morphological and atrid cavity thereby established> 



left side) (Fi'S 14 B' 17) f rom atriopore to posterior end of 



- ' pharynx. C. Folds fused throughout, 



Ihe metapleural loldS enlarge except in the region of the first two 



and new cavities of lymphatic gill A f* s ' which stm open to !j? e " tsi , de - 



J a, Atriopore; gs, primary gill slits; Im, 



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

 i . , right metapleural fold. 



which encroach upon the 



original ccelomic space and lead to their obliteration. 



From the inner face of each metapleural fold a horizontal 



