No. 2.] DEVELOPMENT OF MANICINA AREOLATA. 231 
columella from the theca. The primary mesenteries extend to 
the floor of the depression ; the secondaries end some distance 
above it. The columella is circular in section, and the depres- 
sion surrounds it asa trench. But the trench is not continu- 
ous, being completely divided in the radius of each primary 
septum, as is gathered from Fig. 56, which is taken through 
such a septum. In this figure the septum and columella are 
directly continuous with each other. The columella in reality 
is not solid, as it is drawn in the figure, but is spongy, being 
full of portions of the body cavity which it has cut off during its 
growth. Whether an originally simple and solid columella is 
formed as a central elevation of the basal plate, is an open ques- 
tion. But the subsequent growth of the columella takes place 
by the constant incorporation in it of the lower portions of the 
inner edges of the primary septa. The teeth on the edge of 
the septa, and the granulations found on their sides, sufficiently 
explain the spongy nature of the columella. 
The skeleton inside the polyp is everywhere covered by the 
three layers of the body wall, of which the skeletogenous ecto- 
derm or calycoblast layer is next the skeleton. In Fig. 46 and 
the radial sections these layers are not represented, but they 
are shown in Fig. 51, a more highly magnified portion of one of 
the peripheral entocoelic chambers of Fig. 46. The superficial 
ectoderm is marked ec.; the peripheral parts of the mesenteries 
mes. The calycoblast layer covering the costa, or, more ex- 
actly, the costal tooth, is as elsewhere a layer of flattened cells. 
The skeletal endoderm is likewise very flat, and markedly dif- 
ferent from the endoderm of the body wall and mesenteries. 
In radial sections the calycoblast layer is found to be continuous 
with the superficial ectoderm round the edge of the extra-thecal 
part of the polyp, or vandplatte, R.P. in Fig. 50. 
That portion of the polyp, &.P., that lies outside the theca, 
has been called by Von Heider the randplatte. It has been 
claimed (Moseley, 18; Fowler, 15) that this part of the polyp 
ought not to be regarded as normally on the outer surface of 
the skeleton ; but that when the expanded polyp, which extends 
high above the skeleton, contracts, while most of the body is 
drawn into the interior of the theca, a portion is pulled down 
over the outer surface of the skeleton. However plausible this 
belief may be in the case of those adults in which the extra- 
