LARVAL DEVELOPMENT. 131 



Each gill-slit breaks through in, or slightly to the right 

 of, the mid-ventral line, and then grows well up on the 

 right side of the body. A larva with three gill-slits and 

 the indication of a fourth is represented in Fig. 73. The 

 originally circular mouth has grown to a much larger size, 

 and extends on the left side anterior to the endostylar 



- 73- Larva of Amphioxus, with three gill-slits and the rudiment of a 

 fourth ; from the left side. (After LANKESTER and WlLLEY.) 



/./. Praeoral pit. end. Endostyle lying on right side, seen through the wide 

 lateral mouth, gl. Position of external aperture of club-shaped gland. p.s '. First 

 primary gill-slit, an. Anus. 



N.B. Actual length of larva, nearly iV 2 mm. 



tract (which is on the right wall of the pharynx) and 

 posterior to the first gill-slit. The oral opening later 

 attains to relatively gigantic dimensions, and forms one 

 of the most striking features of the larva. 



The anus is now displaced from its original ventral 

 position to the left side in consequence of the increased 

 development of the provisional caudal fin. The latter 

 consists of elongated ectodermal cells, in which a certain 

 amount of brown pigment is deposited. Later, when 

 the dermal expansion, which has been described above as 

 the definitive caudal fin, begins to grow out, it pushes the 

 cells composing the provisional fin before it, so that they 

 form a fringe round its border. Eventually the provisional 

 fin disappears entirely. 



The gill-slits now go on adding to their number, one 

 after the other, until the larva reaches the stage shown in 

 Fig. 74. In this larva there are fourteen primary unpaired 

 gill-slits, lying, for the most part, on the right side of the 



