ZOOLOGY 
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Within, the cavity of the corallites is divided into a series 
of chambers by the presence of partitions termed tabulae ; 
these may be simple flat plates, but more often they are cup- 
shaped, and in some cases are drawn down in the centre into 
funnel-shaped structures. The tabulae often give off tubes, 
which run out into the platforms. 
The whole skeleton is so compact that it appears to be 
formed of a continuous homogeneous deposit of calcium car- 
bonate. In reality, however, it consists of a number of spicules, 
each with minute serrations which fit ito other serrations on 
the neighbouring spicules. 
The polyp wall is built up of the ectoderm, mesogloea, and 
endoderm. The mesogloea contains a few scattered cells and 
fibres, as well as the skeletal spicules, which, however, in those 
cases where the embryology is known, originate in certain ecto- 
dermal cells which wander afterwards into the mesogloea. 
The tentacles, eight in number, have about fifteen pinnae 
on each side in a single series. They are hollow, their cavities 
Fic. 51.—Transverse section of 
a polypof Tubipora purpurea. 
After Hickson. 
. Stomodaeum. 
. Cilia of siphonoglyph. 
. Ectoderm of stomodaeum and 
of outer surface of body- 
wall. 
4, Endoderm lining mesenteric 
chambers. 
5. Longitudinal muscle fibres in 
mesentery, cut across. 
6, Calcareous spicules, 
7. Parasitic diatoms on the 
ectoderm, 
. Mesogloea. 
Co bh Fe 
Go 
D. Dorsal surface. 
V. Ventral surface. 
opening into the intermesenteric chambers, over which they 
are placed. Their ectoderm is ciliated. The whole body of 
the polyp, including the tentacles, can be withdrawn into the 
corallite. The stomodaeum is ciliated throughout, and on one 
