THE ANATOMY OP THE MADREPORARIA. 
579 
who, in the course of several investigations, has arrived at the 
conclusion that the theca is a secondary structure, derived 
from fusion of the peripheral ends of the septa. The evidence 
adduced in support of this theory appears to me to be at 
present insufficient for complete proof, though from our slight 
knowledge of the group it is injudicious to absolutely deny its 
truth. 
Yon Koch first published this theory in 1879, founding it on 
the following observations on Caryophyllia (4). There is no 
living tissue on the greater part of the exterior of the corallum, 
but at the apex the peripheral edge of the mouth-disc overlaps 
the lip of the calyx in such a way that in the highest sections 
the septa appeared to stand free in the ccelenteron, in sections 
a little lower to have fused peripherally into a theca. The 
costae are, according to him, and as will be seen by the figures, 
the outermost ends of the septa (PI. XL, a, b). 
Further, the mesenteries and chambers between them 
appeared to be continued into this external part of the polyp. 
These appearances he explained by supposing that as the peri- 
pheral ends of the septa approximated and fused they sur- 
rounded the mesenteries, dividing them ultimately into a 
central and a peripheral part. As a further proof he adduced 
the observation that in microscopic sections of the corallum 
sutures were visible in the theca at the points where he 
supposed the septa to have fused. 
I venture to think with Moseley (5) that this explanation is 
erroneous ; that the appearances in the first section (PI. XL, a) 
are due merely to the fact that in this, as in many corals, 
the secretion of calcium carbonate is most active about the 
septa, which consequently rise slightly above the level of the 
theca, as may be seen in any figure of Caryophyllia ; and 
further that, in the second section (PL XL, b), the apparent 
continuation of the mesenteries and chambers between them 
over the tip of the calyx is not due to their having been cut 
into two portions by fusion of the septa, but to more or less 
abnormal contraction due to the use of alcohol; in life the 
polyp, when fully expanded, undoubtedly stretches over the 
