STAGES D AND E, 13 



The appendages arise as hollow processes of the body wall, 

 containing prolongations of the somites. The first to appear 

 are the antennae, into which the prseoral somites are pro- 

 longed. The remainder appear from before backwards in 

 regular order, viz. jaws, oral papillae, legs, 1, 3, . . . 17, and 

 the rudimentary anal papillae, which are the appendages of 

 the last, i. e. the twenty-first somite. 



The full number of somites and their appendages is not, 

 however, completed until a later stage, the posterior being the 

 latest to appear. 



The eyes appear in this stage as invaginations of the sides of 

 the nervous thickenings (the future supraoesophageal ganglia) 

 of the prseoral lobes (fig. 29, e). The invaginations are at first 

 shallow, but soon become deeper, and in the next stage con- 

 verted into closed vesicles, the front wall of which (i. e. the wall 

 next the skin) forms the epithelium outside the so-called lens 

 of the adult eye, while the internal wall thickens, and remains 

 continuous with the cerebral ganglion, and gives rise to the 

 retina. The enclosed vesicle persists, and apparently becomes 

 filled by the structureless lens of the adult eye, if the struc- 

 ture described as such be not a mere coagulum produced 

 by reagents. The eye of Peripatus is therefore a cerebral 

 eye. 



The lips. — The end of the spiral stage is also characterised 

 by the appearance of the buccal fold or fold which encloses the 

 jaws and buccal cavity, and so constitutes the tumid lips of the 

 adult. This is a fold of the side walls of the body immediately 

 outside the jaws, and extending from the prseoral lobe of its side 

 to just behind the jaw. It is at first most marked in front, 

 which fact led Moseley to describe it as a backward process of 

 the prseoral lobe. 



The first indication of the lip is shown in fig. 29, just behind 

 the eye ; it is seen better, however, in the figure of the next 

 stage (fig. 30, L). 



This stage is also characterised by the fact that the anus has 

 shifted to the hind end of the body (the primitive streak 

 having apparently disappeared), The prseoral lobes have also 



