LATER LARVAL STM.KS. 



557 



The lateral folds at first lie very near together on the ventral side 

 of the larva (Fig. 294). They arc found in larvae in which nine or 

 ten primary gill-clefts have developed. Behind this region, the 

 lateral folds are bilaterally symmetrically placed ai the sides of the 

 median line. Near the gill-clefts, on the contrary, they diverge to 

 the righl (Fig. 294). The righl lateral fold (rf) runs forward along 

 the right side <>t' the body above the clefts, in front of which it again 

 bends toward the median line. It covers externally the upper part 

 of the clefts. The left lateral fold (If) is at first only slightly 

 developed anteriorly and runs almost along the median line in the 

 branchial region. 



The lateral folds border a groove running along the ventral side 

 which is the first rudiment of the atrial cavity. The posterior part 



ap 



Fig. 294.— Three larval stages of Amphioxus, from the ventral aspect (after Ray 

 Lamkester and WlLLEY). A. the atrium is still entirely open ; B, the atrium 

 is partially closed posteriori} ; 0, the atrium is almost completely closed, ap, 

 atriopore ; /. , gill-slits; If, left metapleural fold; m, mouth; rf. right metapleural 

 fold : w, pre-oral pit. 



of this groove closes first (Fig. 294 B), closure being effected by the 

 appearance of two solid ridge-like projections of the inner opposed 

 faces of the metapleurs (sub-atrial ridges of Ray Lankester and 

 Willey) ; these ridges grow out towards one another (Fig. 295 

 A, .•>•/) and fuse (Fig. 295 B). Through the fusion of the sub-atrial 

 ridges, the floor of the atrial cavity is formed, in which later that 

 part of the body-cavity culled by Hatschek (No. 8) the cavity of tin 

 lower folds appears (Fig. 311, uf, p. 572). The atrial cavity, which 

 is at first tubular and open at both ends, gradually extends farther 

 forward (Fig. 294 B and C), till, finally, the anterior part of the atrial 

 eavity is completely closed towards the exterior. Only in the posterior 

 part of it do the metapleurs still remain separate. The aperture here 



