192 ARTHUR WTLLE\^ 



are more or less elliptical in shape ; but it will be noticed that 

 the third and fourth^ which are the largest^ are shaped like a 

 plano-convex lens^ their tops being flattened. 



The increase in size of the secondary slits is accompanied by, 

 and in fact is evidence of, a transverse growth by which the 

 primary slits are gradually taken round to the left side. 



The hindermost primary slits are always bent under the 

 ventral wall of the pharynx ; but in fig. 4 this bending under 

 extends to the more anterior gill-slits, namely, up to and 

 including the fourth primary slit. This should be compared 

 with figs. 1, 2, and 3. 



The first primary slit is now smaller than we have hitherto 

 seen it. The twelfth is very small, but when seen in ventral 

 view does not yet show the definite signs of closing which will 

 be described later. 



The anterior wall of the mouth (fig. 25) has now sunk in or 

 bent round so far that it can be easily seen through the right 

 body-wall at this stage (fig. 4). This part of the mouth becomes 

 the right half of the oral sphincter (velum of Huxley and 

 Hatschek). As the mouth sinks towards the right side in this 

 way, so also does the prseoral pit; and the latter gradually 

 becomes flattened out as the development of the oral hood pro- 

 ceeds, and eventually becomes simply a ciliated tract on the 

 under side of the oral hood, known in the adult as the " Rader- 

 organ,^' which has been identified by Hatschek with the praeoral 

 pit of the larva. 



The club-shaped gland and endostyle are approximately in 

 the same condition as in the preceding stages, but in some 

 larvse of this stage I detected signs of the backward growth of 

 the endostyle, in that the club-shaped gland was frequently 

 seen to overlap the posterior edge of the endostyle. 



Of the six secondary slits which have been described above 

 as appearing at the same time, the first, which nearly always 

 lies between the third and fourth primary slits, does not 

 eventually become the first secondary slit, but it becomes the 

 second, a new one being formed in front of it. Tlie latter, 

 however, occasionally appears as soon as tlie others. Thus 



