536 INVEETEBEATA CHAP. 



METAMORPHOSIS OF THE AURICULAlilA 



Signs of the metamorphosis now appear. The hydrocoele is still 

 in the form of a vertical hoop with its concavity directed towards the 

 right. From the anterior edge of the left posterior coeloni a finger- 

 like process grows out and extends along the hydrocoele. This is 

 the peri-oral coelom (p.o.c, Fig. 398), homologous with the peri-oral 

 coeloin in Asteroidea and Ophiuroidea. At the same time the left 

 and right posterior coeloms approach each other ventral to the 

 intestine, and eventually become applied to one another in an oblique 

 line. The left coelom extends over to the right in front of the right 

 coelom ; this extension is evidently homologous to the right ventral 

 horn of the left posterior coelom in the larva of Asterina gibbosa. 

 Then the longitudinal ciliated baud breaks into fragments, owing, no 

 doubt, to the fact that the neighbouring ectoderm grows more quickly 

 than that forming the band. 



The following are the pieces into which the band breaks up : The 

 prae-oral loop gives rise to a median and two lateral pieces (Fig. 

 399, A), and the anal loop gives rise to three precisely similar pieces. 

 Each side of the longitudinal band breaks up iuto four pieces, three 

 of which roughly correspond to the antero-dorsal, intermediate 

 dorsal, and postero-dorsal larval arms. The fourth piece corresponds 

 to the re-entrant angle between intermediate dorsal and postero-dorsal 

 arms. Finally, each postero-lateral arm gives rise to a separate 

 fragment of the band. 



Semon maintains that even before this stage a larval nervous 

 system exists (l.nerv, Fig. 398), consisting of two longitudinal ridges 

 of thickened ciliated ectoderm which are independent of, and within 

 the field embraced by, the sides of the larval ciliated band. Semon 

 draws the conclusion that these are nervous, because in sections he 

 finds that there are fibres at their bases ; but no separate nerve-cells 

 have ever been discovered. According to Semon, as metamorphosis 

 proceeds, these bands become carried into the interior of the larva by an 

 invagination which involves the mouth and form the adult nerve-ring. 



Bury gives a different account of the matter, and as Semon has 

 proved to be inaccurate in other points we follow Bury. This author 

 points out that by inequality of the rates of growth of neighbouring 

 parts the mouth is carried over to the left side, and the apical pole 

 of the larva is displaced to the right. Then a ciliated ring 

 round the mouth is formed by the juxtaposition of four pieces of the 

 old longitudinal ciliated band : these four are the median pieces of 

 the prae-oral and anal loops and the pieces of the sides of the band 

 mentioned above, which correspond to the re-entrant angles between 

 intermediate-dorsal and postero-dorsal arms on each side (Fig. 398). 



Then the invagination described by Semon occurs, by which the 

 larval mouth surrounded by the circular ring comes to lie at the 

 bottom of a deep pit, the aperture of which nearly but not quite 

 closes ; the opening of this atrium is distinctly on the left side. The 



