THE DEVELOPMENT OF ALCYON1UM DIGITATUM. 
85 
endodermic portions, the endodermic being well developed in 
the ventral, and much less so in the dorsal. Growth seems 
slower in the dorsal than the ventral filaments, so that by the 
end of the seventh day they are still much shorter than the 
latter (PI. 4, fig. 26). 
Sections made of polyps for the examination of the fila- 
ments at this stage, also show that the retractor muscles of 
the mesenteries are now developing. By the thirteenth to 
fifteenth day of sedentary life the ventral filaments are 
twice as long as the dorsal, although they extend very little 
below them, because of their convoluted condition. By this 
date, also, the filaments approximate more nearly to the 
adult in transverse section, i.e. the ciliated surface of the 
dorsal filament has become more concave, while the glandular 
ectoderm of the ventral has become more convex, and extends 
further round (cf. Text-figs. 49 and 51 with PI. 4, fig. 29,. 
and ( 3 ) PI. 38, figs. 18 and 19). 
Summary of other Writers’ Yiews on the Derivation of 
the Mesenteric Filaments, and Remarks on these. 
H. Y. Wilson ( 15 ) considers that the ventral mesenteric 
filaments in the coral Manicina are wholly ectodermic in 
origin, and gives a very similar figure to the present PI. 4, 
fig. 33, showing the down growth from the stomodgeum of the 
ectodermic bands which give rise to the filaments. E. B. 
Wilson ( 17 ) states that the dorsal filaments in colonial polyps 
of Alcyonium are of ectodermic origin, but while he shows 
( 16 ) that endoderm certainly enters into the ventral filaments 
of Renilla, he did not find any ectodermic outgrowth con- 
tributing to them. It is not impossible that he passed over 
this stage in development. J. Stanley-Gardiner ( 13 ) con- 
siders that the ventral mesenteric filaments of Coenopsammia 
are purely ectodermic, basing this view on histological 
grounds. Also he mentions the same fact for Flabellum ( 14 ). 
It is possible that investigations into the early development 
of other Anthozoa would confirm the fact that all the fila- 
ments throughout the group consist of ectodermic and endo- 
