148 



DEVELOPMENT OF AMPHIOXUS. 



The prseoral pit itself is absorbed, as it were, into the 

 oral hood, so that it eventually loses its independent exist- 

 ence as a pit, although the sense-organ of the praeoral pit 

 persists in the adult as a deep groove in the dorsal wall 

 of the oral hood to the right of the base of the notochord. 

 The remaining ciliated epithelium of the original praeoral 

 pit increases in extent, and grows out into the finger- 

 shaped tracts which we have already described as being 

 characteristic of the inner surface of the oral hood, consti- 

 tuting the so-called " Raderogan." (Cf. Fig. 3.) 



Equalisation of the Gill-slits. 



In the stage next succeeding that of which a ventral 

 view is given in Fig. 80, the first eight primary slits that 

 is to say, from the original second to the ninth inclusive - 



Fig. 84. Larva toward the close of the metamorphosis, from the left side. 

 (After WILI.EV.) 



o. Olfactory pit. v. Velum, p.b. Peripharyngeal band. e. Endostyle. /.A Second 

 primary slit, the first having closed up. m. Left metapleur. s.a. Floor of atrium. 

 p,s vl . p.s 13 . Vestiges of the twelfth and thirteenth primary slits. 



have become definitely established on the left side, their 

 longitudinal and vertical axes are equalised, and in most 

 of them the tongue-bars are completely formed (Fig. 84). 

 No tongue-bar is formed in the first slit on either side, and 

 this slit apparently remains as a rule simple throughout 

 life. 



