Fig. 8

Schematic drawing of paired gill perforation in amphioxus. a Transverse section showing opening of branchial nephridium into pharynx (arrowhead) at which gill perforation occurs at the dorsal end of atrium (arrow). b Gill opens through coelomic mesothelia on both ectodermal and endodermal sides (arrow). This gill formation is comparable to mouth formation. Redrawn from ref. 50. at, atrium; bn, branchial nephridium; c, coelom; ge, gill epithelium; gp, gill pouch; gs, gill slit; m, myomere; ph, pharynx