No. 2.] DEVELOPMENT OF MANICINA AREOLATA. 229 
polyp in such a way as to divide the cavity and mesenteries into 
central and peripheral parts. This conclusion, though not uni- 
versally accepted, is, I think, now incontestable. Von Heider 
in 1882 (14) published the important observation that the skele- 
ton is not only covered by supporting lamella and endoderm, 
but that between the calcareous matter and the supporting 
lamella is another layer of cells, to which was given the name 
of calycoblasts. Von Koch in 1883 (13) made the whole matter 
clear by showing that in the young Asteroides the skeleton is 
secreted by the ectoderm and is at first entirely outside the 
body. The ectoderm of the basal surface of the attached 
larva secretes a calcareous basal plate. Radial folds of the 
basal ectoderm then grow up between the mesenteries into 
the cavity of the polyp, lifting up, as they grow, the supporting 
lamella and endoderm. The cavity of the fold is filled with 
calcareous matter, which is the septum. The secreting ecto- 
derm becomes the calycoblast layer of Von Heider. The origin 
of the basal plate and septa is thus clear enough, but Von 
Koch’s account of the development of the theca is not satis- 
factory. As far as his observations went they appear to have 
confirmed his belief, gathered from a study of adult corals, that 
the theca is formed secondarily from the septa; the originally 
simple septa become bifurcate at their peripheral ends; the 
lateral processes of adjacent septa grow towards each other, 
pierce the mesentery, and, fusing, form the theca. Von Koch’s 
account of the relation of the skeleton to the soft parts in the 
adult has recently been confirmed on a number of genera by 
Fowler (15) and Bourne (16), though as regards the extra-thecal 
part of the ccelenteron Fowler differs from Von Koch in the 
interpretation of his sections. From these papers it is also 
evident that the Madreporaria exhibit great variety in the 
details of the relation between the soft parts and skeleton. 
My own very incomplete observations on the skeleton of Manz- 
cima are for the most part a confirmation of Von Koch’s state- 
ments. 
In the newly attached larva, Fig. 37, the ectoderm of the 
basal surface is made up exclusively of supporting cells. The 
granular cells present at the aboral end of the swimming larva 
have all disappeared. Some time after attachment a small patch 
of calcareous matter is found on the basal surface, and sections 
