228 WILSON. [Vot. II. 
ment is ectodermal. Though the filament of the adult coral is 
a much simpler form than the trifid filament, it would be diffi- 
cult to derive the latter from the former, owing to the presence 
of mesenteric lobes in the coral. It is easy, however, to derive 
the trifid from the larval filament, Fig. 35. In this form the 
ciliated tracts, v./, and the median secretory tract, are already 
differentiated ; to produce the trifid filament it is only necessary 
for these tracts to become separated by the division of the 
mesoderm into three lobes. By this division, while the median 
stripe gains but little, the ventro-lateral tracts are put in a 
position where it is possible for them to reach a high develop- 
ment. 
The number of Actinozoa in which the mesenterial filaments 
have been carefully studied is very limited, but from the data at 
hand it seems probable that the ancestral filament, like the 
filament of the larval M/anzcina, was a simple undivided body, 
in which, however, the originally uniform ectoderm had become 
split up into three physiologically distinct tracts : a median tract, 
in which were concentrated the nettle and glandular elements, 
and which embraced most of the filament; and two lateral 
ciliated portions which were but slightly developed. The belief 
that the division of labor in the Zoanthorian filament dates very 
far back, is supported by the existence of a similar division of 
labor in the Alcyonaria. But in these polyps, as Wilson (8) has 
shown, the functions are not distributed over different parts of 
the same filament, but amongst the several filaments. The 
dorsal pair are ciliated bands, having no gland or nettle cells, 
while the remaining six filaments contain gland and nettle cells, 
but have no ciliated tracts. 
IX. Tue SKELETON. 
Until the time of. Von Koch’s researches the skeleton of the 
Madreporaria was regarded as calcified mesoderm. The theca, 
or coral wall, according to this view, represented the supporting 
lamella of the lateral body wall of an actinia; and where, as in 
most corals, the theca was largely uncovered by animal sub- 
stance, the explanation was that the ectoderm had atrophied. 
In 1879 Von Koch (12) showed that the theca is independent of 
the lateral body-wall, and projects into the ccelenteron of the 
