DEVELOPMENT 47i 



mentof the Lancelet is rapid. In a few hours after fertilisation 

 the gastrula may be seen rotating within the still unbroken egg- 

 envelope ; in eight hours it is hatched or set free ; it swims 

 about for twenty-four hours or so while its essential organs are 

 being laid down ; about the thirty-sixth hour it gets an open 

 mouth and anus and a gill-cleft, and we call it a larva. 



The young larva (Plate XXXIV., C) has a ciliated ecto- 

 derm, like a young tadpole and is more active than later stages. 

 One of its peculiar features is the lack of symmetry — the 

 mouth is to the left side, the gill-slits are to the right, the anus 

 is median. Later on this asymmetry is considerably lessened, 

 but there are many expressions of it in the full-grown animal. 



