316 ON THE EDWARDSIA-STAGE OF LEBEUNTA. 



Fig. 20. Section towards the aboral region. Stages in the disorganization of 

 the central tissue are represented. 



Fig. 21. Vertical section through a larva of the same stage, such as would be 

 obtained along the lines I-x, x-1 in fig. 19. The section passing 

 through the pair of mesenterial filaments shows their continuity 

 with the lining of the archenteron. 



Fig. 22. Transverse section through the free edge of the sulco-lateral pair of 

 mesenteries of a larva one or two days old, taken a little below the 

 oesophagus. The mesentery to the left shows the beginning of the 

 formation of the mesenterial filaments, while that to the right is as 

 yet unaltered. A few sections lower the filament appears strongly 

 developed on each. X 320. 



Figs. 23-25. Transverse sections selected at difi'erent levels from a com- 

 plete series through a larva several days old, showing the relation of 

 the plane of symmetry of the tentacles to the sulco-sulcular plane of 

 the larva. X 50. 



Fig. 23. Section through the tentacular region and the oesophagus. The 

 tentacles towards their origin are in the same bilateral relation 

 as in fig. 7. The mesenterial chambers are further developed than 

 in any previous section represented. The larger axis of the 

 oesophagus is at right angles to the dorso-ventral axis of the 

 tentacles. 



Fig. 24. Section towards the lower end of the oesophagus representing still 

 the same relations. The sulco-sulcular axis of the mesenteries (a-e) 

 and the longer diameter of the oeiophagus are at right angles to the 

 axis of symmetry of the tentacles (g-c). 



Fig. 25. Section below the oesophageal region and the tentacular prominences. 

 The axes are in the same relation as in the two previous figures. 

 The axis of symmetry of the mesenteries is at right angles to that 

 corresponding with the dorso-ventral axis of the tentacles. 



Figs. 26-28. Transverse sections through a slightly compressed larva of about 

 the same age as the former. The series show that the sulco-sulcular 

 plane of the larva (a-c) is oblique to the dorso-ventral plane of 

 the tentacles {f, b). X 50. 



