604 INVERTEBRATA CHAP. 



the number of these is reduced to eight. A new series of eight gill- 

 slits make their appearance on the right side of the larva, above the 

 primitive gill-slits. These new gill-slits (termed by Willey the 

 secondary gill-slits) grow downwards, pressing the primitive gill- 

 slits back to the mid-ventral line of the larva, and eventually on to 

 its left side. Thus the "primary" gill-slits form the adult gill- 

 slits of the left side, and the " secondary " gill-slits those of the 

 right side. In this way the lower borders of the primary gill-slits 

 become the dorsal borders, and the excretory organs reach the 

 position in which they are found in the adult, vix. the dorsal border 

 of each gill-slit. 



At the same time, in both primary and secondary gill-slits, septa 

 grow out from their dorsal edges, extend across them, and eventually 

 divide each of them into two. These septa are the tongue-bars (t!b, 

 Fig. 441). The endostyle, which like the gill-slits lay originally on 

 the right side of the pharynx, is forced downwards to the mid-ventral 

 line ; it grows backwards until it extends along the whole length of 

 the pharynx. The buccal cavity or stomodaeum is formed by the 

 outgrowth of two cheek-like folds from the collar region (region of 

 the first myotome), which extend forwards beneath the notochord. and 

 enclose between them a space which formerly was part of the external 

 world, lying in front of the larval mouth. The larval mouth persists 

 at the bottom of the stomodaeum as an opening in the so-called velum, 

 leading into the pharynx. 



On the dorsal surface of the buccal cavity an extensive series of 

 ciliated grooves makes its appearance. These grooves radiate from 

 the opening of the left head-cavity, now termed Hatschek's pit, and 

 their function seems to be to carry off the secretion of this pit. They 

 are termed collectively the wheel-organ (-w.o, Fig. 441). At the 

 side of the buccal cavity the ciliated rods, known as oral cirri, grow 

 out (Fig. 441, C). The atrial ridges unite to form the floor of the 

 atrial cavity ; on their inner surfaces ectodermic thickenings appear 

 which are believed to be excretory in function, and are termed 

 Miiller's papillae (Fig. 441, B). 



The metamorphosis may now be regarded as complete, and the 

 animal takes up its habitat in a vertical burrow in the sand. In this 

 burrow it leads a sedentary life, only occasionally emerging from it 

 to make a rapid dash through the water and then seek a new burrow. 



The young Amphioxus has still to produce new gill-slits, since, it 

 has only eight pairs to start with ; and it has only the rudiment of a 

 liver. As the animal increases in size, new gill-slits gradually 

 appear behind the existing gill-slits on each side, and a full-grown 

 specimen may have as many as 100 pairs. Each gill-slit begins as a 

 simple perforation which is then bisected by the growth of a tongue- 

 bar. The liver begins as a little ventral pouch of the gut immediately 

 behind the pharynx. This pouch grows forward and becomes pushed 

 up on to the right side of the animal. Eventually it becomes so long 

 that its tip reaches nearly to the velum. 



