LARVAL DEVELOPMENT. 149 



In Fig. 84 the last indications of the twelfth and thir- 

 teenth primary slits are to be observed as slight depres- 

 sions in the floor of the pharynx in the mid-ventral line. 

 The tenth and eleventh slits would close up later. 



It should be pointed out that the closure of the poste- 

 rior primary slits does not proceed in perfect correspond- 

 ence with the age of the larva, but takes place sometimes 

 at an earlier and sometimes at a later stage than here 

 depicted. 



The gill-slits of both sides now begin to elongate in 

 the vertical direction (Fig. 93), and eventually a very well- 

 marked stage is reached, which is characterised by the 

 presence of eight pairs of gill-clefts. This latter stage 

 would appear to have a considerable duration, and, as it 

 stands on the borderland between the larva and the adult, 

 and forms the commencement of what may be called the 

 adolescent period of the development, it may well be 

 regarded as a critical stage. By this time the young 

 Amphioxus has given up its free pelagic life in the open 

 sea, and has commenced to burrow in the sand, which it 

 continues to do for the rest of its life.* 



Further Growth of Endostyle, etc. 



At the point at which we left the endostyle, its two 

 halves were in the relation to one another of upper and 

 lower. During the steps in the metamorphosis which we 

 have recorded above, the upper half of the endostyle is 

 brought down to the same level as the lower half on the 

 right side of it, and so the definite form of the endostyle 

 is established by the conjunction of its right and left 

 halves. It then proceeds to grow backwards along the 



" The duration of the larval development of Amphioxus may be estimated 

 at about three months. 



