554 



CEPHALOCHOBDA. 



secutive connected oval thickenings. This ridge, which represents 

 a thickening of the pharyngeal wall, is the rudiment of a new row of 

 gill-clefts, the secondary clefts of WILLEY which remain on the right 

 side of the body. Their true character is soon announced by fusion 

 of the entodermal thickenings with the ectoderm and by the ap- 

 pearance of small elongated perforations in the middle of the ovals 

 (Fig. 292 B, I-VII). The rudiments of six secondary clefts usually 

 first appear, alternating in position with the primary clefts in the way 



fH 



dr es 1 



FIG. 292. Two larval stages of Amphioxus, seen from the right side (after WILLEY)- 

 1, 2, 9, 12, 14, first, second, ninth, twelfth and fourteenth primary gill-clefts- 

 1- VII, first seven secondary gill-clefts ; an, eye-spot ; ch, chorda ; dr, club- 

 shaped gland ; es, rudiment of endostyle ; fh, dorsal fin-cavities ; ///, ventral fin- 

 cavities ; m, margin of the mouth ; mf, edge of the right metapleural fold ; k, 

 rudiment of the secondary gill-clefts ; n, medullary tube ; p, atrial cavity ; si, 

 branchial artery ; v, velum ; w, pre-oral pit. 



described above in connection with the primitive segments of the 

 right and left sides of the body (p. 545). The most anterior of these 

 six secondary gill-rudiments lies between the third and fourth primary 

 clefts. The number of secondary clefts increases later, one more 

 forming anteriorly, and several being added posteriorly, so that, 

 finally, they number in all seven to nine, eight being the most 

 common number. 



The later changes in the branchial region consist of the shifting 

 of the row of primary gill-clefts over the ventral surface of the 

 pharynx from the right side of the body to the left. The oral 



