232 WILSON. (VoL. II. 
thecal part of the polyp is confined to the upper edge of the 
theca, it is at once found to be untenable on examining a small 
number of young JZanicinae of the size of Fig. 50. In the 
polyp, for instance, from which this section was made, the rand- 
platte covered more than one-half of the lateral surface of the 
skeleton, and this was true both in the expanded and contracted 
condition of the animal. Further, in every dozen such young 
Manicinas, one or two asymmetrical ones will be found, in 
which, while the randplatte is confined to the upper half of the 
lateral surface on one side of the polyp, on the other side it 
covers the whole wall down to the surface of attachment. What 
puts the matter beyond dispute, in this genus at least, is that in 
the very young Manzicina, Figs. 41 and 42, the whole skeleton 
is practically inside the polyp. I also found two or three older 
specimens (single polyps) in which the entire lateral surface of 
the skeleton was covered by the randplatte. One of these which 
I sectioned, was oval in transverse section, long axis about ? in.; 
short axis 1in. The skeleton had a flat surface of attachment, 
and was } in. in height. The randplatte (on rather extra-thecal 
polyp) in this individual was unbroken, down to the piece of 
rock to which the coral was fastened; here its ectoderm turned 
in to form the calycoblast layer. 
It appears, then, that up to a certain age, which varies much 
in individuals, the lateral surface of the skeleton is entirely cov- 
ered by the polyp. The transformation of the originally large 
extra-thecal part of the polyp into the relatively small rand 
platte may possibly take place in many cases gradually, by the 
constant dying off of this part of the polyp at its free edge, and 
subsequent disappearance of the dead tissue. But in some in- 
stances this is not the case. I found a number of small single 
polyps in which I first thought the randplatte covered the en- 
tire lateral wall of the skeleton, but on looking again, I saw that 
a very definite line extended all round the lateral surface at 
about the level of the edge of the randplatte in Fig. 50. 
Above this line the soft parts looked perfectly healthy, but be- 
low it dull and sickly. On sectioning I found that above the 
line the randplatte was normal, but that the tissue below it was 
a membrane with scarcely a trace of cellular structure and 
made up for the most part of plant filaments. In some asym- 
metrical specimens the portion of the skeleton left uncovered by 
