132 



DEVELOPMENT OF AMPHIOXUS. 



pharynx, although the more posterior slits bend under the 

 pharynx, while the most posterior have a median ventral 

 position. 



In front the gill-slits still open directly to the exterior, 

 but the right metapleural fold is seen to be hanging over 

 the tops of them ; while the hinder slits now open into 

 the partially formed atrium, which has already closed in 



Fig. 74. Anterior portion of larva, with fourteen primary gill-slits and rudi- 

 ments of the secondary gill-slits ; viewed as a transparent object from the right 

 side. (After WILLEY.) 



s.o. Sense-organ of prasoral pit (groove of Hatschek). e. Endostyle. gl. In- 

 ternal opening of club-shaped gland, s.s. Rudiments of secondary gill-slits. p.s iA , 

 /.J 14 . Thirteenth and fourteenth primary gill-slits. The lower margin of the 

 mouth is seen through the anterior gill-slits. 



Total length of larva, nearly 3% mm. 



posteriorly, as described above. The larva is remarkably 

 transparent, so that its internal organisation can be seen 

 as clearly as possible through the outer body-wall. 



The long axis of the primary gill-slits is approximately 

 at right angles to the long axis of the body. They are 

 not more numerous than the myotomes in the correspond- 

 ing region of the body, so that the branchiomery at this 

 stage coincides with the muscular metamery. In Fig. 73 

 the first gill-slit was somewhat larger than the second and 

 third. At about that stage, however, its further growth 

 became arrested, and now it is seen to be considerably 

 smaller than those which immediately follow it. 



In addition to its external opening on the left side, be- 



