THE DEVELOPMENT OP ASTBEINA GIBBOSA. 239 



adult plane makes an angle of about 70° or more with the 

 larval plane ; but without any very serious error, it may be 

 regarded, for purposes of description, as at right angles to it : 

 thus the direction right to left, according to the larval plane, 

 becomes aboral to oral according to the adult plane, and dorsal 

 to ventral according to the larval plane is nearly parallel to 

 the adult plane. Here I may remark that the words "dorsal" 

 and "ventral" will only be used with reference to the larval 

 plane ; in speaking of the adult plane the words " oral " and 

 "aboral" will be used. 



PI. XI, figs, la and 13, show the appearance of a larva which 

 has only been fixed for a short time. On the left side we see 

 that the hydrocoele lobes have become visible externally, since 

 they have raised the ectoderm into protrusions which, as we 

 shall find, are the rudiments of sensory terminal tentacles of 

 the radial water-vascular canals. Outside the curve of these 

 rudiments is another set of protrusions, also arranged in an 

 open curve. These are the rudiments of the arms : they are 

 all, as we shall see, outgrowths of the left posterior coelom, and 

 their primary function is to form supports for the lobes of the 

 hydrocoele, to which they later become apposed. The con- 

 striction of the prseoral lobe or stalk from the body proper is 

 hardly as yet marked, but the rounded appearance of the dorsal 

 and ventral outgrowths of the prseoral lobe is to be noticed. 

 This is due to the disappearance of the larval organ, the opposite 

 sides of which become approximated to each other and wrinkled, 

 and then broken up, portions of the organ becoming invaginated 

 into the interior and destroyed by histolysis. The appearance 

 of the remnants of it at this stage gave Ludwig the impression 

 that one had to do with the outgrowth of a series of protrusions 

 homologous to the adhesive disc. This is, of course, a mistake; 

 the adhesive disc remains single and unaltered to the end of 

 the metamorphosis. This well-marked phase of development 

 we may call Stage E. PI. XIII, figs. 48 to 50, are taken from 

 a larva of this age j fig. 48 is of course the most dorsal section 

 (see explanation of plates). In fig. 50 we notice the great 

 growth of the left hydrocoele, lobe 3 reaching nearly to the 



