282 ME. J. E. DUEEDEN OK 



the adult condition is for the archenteron to extend itself a 

 little, and thus set up a communication with the exterior. 



Vertical sections of one of the freshly-expeUed larvae prove 

 that this is apparently not the case, but that to form the adult 

 oral aperture and oesophagus a distinct invagination of the super- 

 ficial ectoderm takes place. This is very distinctly shown in the 

 actual sections represented in PI. 18. figs. 9 to 11, taken from a 

 series of vertical sections through the archenteric region of an 

 extruded larva, before disintegration of the central tissue has 

 commenced. In the particular larva the transverse, separating 

 tissue of the invagination has evidently just broken through, 

 establishing communication, but the lateral walls still indicate 

 how the whole process has been carried out. 



Eig. 9 is from a tangential section through the wall of the 

 archenteron and the invaginated region above. The ectoderm 

 directed inwardly differs much in character from that beyond ; 

 zooxanthellae are absent, and a greater number of deeply-staining 

 nuclei cause it to stand out very distinctly. A straight, ciliated 

 edge indicates the free surface of the floor of the ectoderm ; 

 while a middle region below, nearly devoid of nuclei, represents 

 where the lower surface of the invaginated end has come into 

 contact with the expanded upper end of the archenteron. 



The free margin of the wall of the latter, bordering on the 

 flattened archenteric or coelomic chamber, is also very strongly 

 ciliated. The floor of this chamber is at this stage entire and 

 sharply defined ; the limiting layer is ciliated and contains 

 abundant nuclei, which easily distinguish it from the vacuolated 

 tissue below and around. To the right the floor is continued 

 furtber into the diverticular system {cf. left side of fig. 15, PL 19). 



Eig. 10 (PL 18) is from another tangential section, but nearer the 

 lumen than the former. An external aperture is now presented, 

 the outer ectoderm being seen as an inturned layer. The floor 

 of the invagination is dome-shaped and extends some distance 

 laterally. The area of its union with the roof of the archenteric 

 cavity can even yet be distinguished owing to its diminished 

 number of nuclei. The central coelomic space is still flattened, 

 but the preparation does not permit of the lateral extensions 

 being followed. 



Eig. 11 represents a section through the actual lumen. The 

 central tissue, consisting of the floor of the ectodermal invagina- 



