xm PHYLUM CHORDATA 61 



dorsal to them, a second longitudinal series makes its appearance, 

 containing eight clefts, so that at this stage there are two parallel 

 rows of gill-slits on the right-side of the body, and none on the 

 left. But as growth goes on, the first or ventral series gradually 

 travels over to the left side, producing a symmetrical arrangement, 

 and at the same time the first slit and the last five of the first or 

 definitively left series close up and disappear, so that the numbers 

 are equalised on the two sides. At first each gill-slit is simple, but 

 before long a fold grows down from its dorsal edge, and, extending 

 ventrally, divides the single aperture into two : this fold is the 

 secondary or tongue-lamella, the original bars of tissue between the 

 undivided slits becoming the primary lamellae. 



While the development of the gill-slits is proceeding, the atrium 

 is in course of formation. Paired longitudinal ridges, the meta- 

 pleural folds (Fig. 764, If. rf.. Fig. 765, sf.), appear on the ventral 

 side of the body, behind the^gill-sHts, and^gradually extend forwards, 



ap 



FlQ. 764. Amphioxus lanceolatus. Ventral aspect oi' three larvae showing the develop- 

 ment of the atrium, ap. atriopore ; k. gill-slits ; //. left metapleural fold ; m. mouth ; rf. right 

 metapleural fold ; w. pre-oral pit. (From Korschelt and Heider, after Lankester and Willey.) 



dorsal to the latter, their arrangement being very unsymmetrical 

 in correspondence with that of the clefts themselves. On the 

 inner face of each fold, i.e. the face which looks towards its fellow 

 of the opposite side, a longitudinal sub-atrial ridge (Fig. 765, A, si) 

 appears, and the two sub-atrial ridges meeting and coalescing, a 

 canal (B, p) is formed immediately below the ventral body-wall. 

 This canal is the commencement of the atrium : it is at first quite 

 narrow, but gradually extends upwards on each side (C, p) until it 

 attains its full dimensions. It is open, at first, both in front and 

 behind : the posterior opening remains as the atriopore : the 

 anterior opening becomes gradually shifted forwards as the fusion 

 of the sub-atrial ridges proceeds (Fig. 764, B and C), and is finally 

 completely closed. In this way the gill-slits come to open, not 

 directly onjthe exterior, but into a cavity formed by the union of 

 paired ridges of the body-wall, and therefore lined by ectoderm. 



