THE ANATOMY OF THE MADREPORARIA. A()7 
While my sections do not begin till the plane of the septa is 
reached, there can be no doubt (and it seems likely from 
Professor Moseley’s MS. sketch) that the soft tissues are con- 
tinued on the inside to the summit of the epitheca, in the 
manner represented in the diagrammatic fig. 2, since growth 
of that part would not otherwise be possible. Passing inwards 
from this point the oral disc slopes downwards to the position 
of the strong endodermal sphincter muscle (sph., fig. 2) by the 
agency of which the disc is drawn down over the tentacles. 
Sections a little way below this point show the tentacles lying 
in a continuous space bounded both centrally and peripherally 
by ectoderm,! i.e. in the pouch of ectoderm formed on the 
contraction of the sphincter (figs. 2, 3). Lower yet, the 
columella and its pillars are cut before the bottom of the 
stomodzum is reached, the latter being thrown into tortuous 
folds among them. The tentacles are simple, and entoccelic 
only, i.e. twenty in number; their ectoderm is not differen- 
tiated into batteries, but is charged with large nematocysts. 
The mesenteries amount to twenty pairs, of which, like the 
septa, ten are longer than the rest; they all bear filaments, 
and two of the longer pairs at opposite ends of a diameter of 
the circular polyp are “ directives ;’ their mesoglea lamina is 
very thick and coarsely pleated, the pleats showing no tendency 
toarborescence. The mesoglcea, indeed, is unusually thick over 
the greater part of the polyp. The endoderm is composed of 
very long vacuolated cells, so long as almost to obliterate the 
peripheral parts of the mesenterial chambers; the filament is 
of the usual character, the ova are, in structure and relations, 
1 This appearance in transverse section must be carefully distinguished, in 
investigating Madreporaria, from a somewhat similar one, which is due merely 
to distortion, and in which an excessive contraction of the mesenteries (cf. 
von Heider, ‘ Zeit. wiss. Zool.,’ xliv, p. 158) has pulled the oral disc into 
pockets, so that a space bounded internally by ectoderm appears to lie either 
in the centre of the mesenteries or in the mesenterial chambers ; in the latter 
case, the cross-section of a tentacle is often found in the centre of the space. 
The tentacles then lie, however, in a series of pockets; in the case of 
Dunceania, &., the pouch in which they are found is a continuous circle 
(ef. fig. 3). 
