38 PHYLUM CEPHALOCHORDA. 



the atrial cavity (Fig. 23). At their hind end they remain 

 separate, thus giving rise to the atriopore. At first the atrial 

 cavity is a small canal restricted to the ventral side of the 

 pharynx. Later it becomes larger and acquires the adult 

 relations. 



Meanwhile a row of eight or nine gill -clefts appear on the right 

 side of the body dorsal to those first formed (Figs. 24, 25). Both 

 sets of gill-clefts acquire the U-shaped form, the tongue bar being 

 developed (except in the first cleft (Fig. 25) which remains 

 simple). The first-formed clef ts then shift on to the left side of 

 the body, and become the gill-clefts of the left side. At the 

 same time the mouth shifts to the middle line, and the preoral 



ZT 



FIG. 23. Ventral view of three larval stages of Amphioxus (after Lankester and Willey* 

 from Korschelt and Heider) ; A, the atrium is still entirely open ; B, the atrium is partially 

 closed behind ; C, the atrium is almost completely closed ; ap atriopore ; k gill-slits ; 

 If left atrial fold ; m mouth ; rf right atrial fold ; w preoral pit. 



hood which had made its first appearance while the mouth was 

 still on the left side becomes developed. The apertures of the 

 club-shaped gland (see below) and of the ciliated pit are both 

 enclosed by the preoral hood. The original mouth opening 

 shifts to the back of the oral cavity and persists as the aperture 

 in the velum. 



The principal phases of the development are now accom- 

 plished and the larva, in the main symmetrical, assumes the 

 sand-burrowing habits characteristic of the adult. 



The mesoblastic somites, after their separation from the 

 archenteron, which after that event is termed simply enteron, 

 extend ventralwards on each side (Fig. 26), till they meet on the 



