METAMORPHOSIS OF ASTERTNA GIBBOSA. 229 



a very late stage of metamorphosis, found on the right side of 

 the body. This is brought about apparently by the greater 

 atrophy of the right side of the preoral lobe. In Asterina gibhosa 

 this unequal atrophy of the two sides of the preoral lobe becomes 

 manifest in a much earlier stage, when the lobe has scarcely 

 undergone any shrinkage. As a consequence of this the pre- 

 oral lobe is bent towards the right side of the body, and the 

 aboral disc never makes a right angle with it, as it does in 

 Ästerias pallida. This is very evident in the figures of Ludwi«- 

 and MacBride, and is also shoAvn in fig. 14, PL XVIir, of the 

 present paper. The maximum inclination of the aboral disc 

 with respect to the preoral lobe is, in the present species, 60°-70°, 

 as has been pointed out by MacBride, and as may be seen in fig. 

 13, c & d. From this maximum inclination on, the lobe makes less 

 and less an angle with the aboral disc, inasmuch as it is beut more 

 and more towards the right side of the body as metamorphosis 

 progresses (fi.g. 14). But does this show the method of orienta- 

 tion developed in my paper on Asterias to be inapplicable to 

 the present case ? I think not. Because exactly the same process 

 takes place, as I have remarked above, in Asterias, but in a 

 much later stage. If we look closely into the matter we shall 

 see that there are here two forces at work in difierent directions, 

 one tending to bring the aboral disc and the preoral lobe to 

 such a position that they make a right angle with each other, 

 and the other tending to diminish the angle between them. In 

 Asterias pallida the latter force comes into play only at a very 

 late stage of metamorphosis ; but in Asterina gibbosa its effect 

 becomes pronounced at an earlier stage, and thus disguises the 

 real state of things that would otherwise be manifest. In both 

 cases the plane of bilateral symmetry is bent on itself at the 



