STRUCTURE OE A SPECIES OE MILLEPORA. 
127 
over the calicle nearly or sometimes completely closes it, and thus usually the zooid 
can be seen in preparations in which the hydrophyton is viewed from the surface only 
by focusing the objective into the depths of the tissue. The orifices of the calicles of 
retracted zooids were unfortunately not carefully examined in the living condition of 
the coral ; hence it is uncertain whether the superficial tissue contracts in this manner 
in the living condition so as to close the orifice of the calicle and protect the zooid, or 
whether such extreme contraction occurs only through action of reagents. It seems 
probable that it does occur in the living animal, since by its means the masses of large 
thread-cells are brought as a protection directly between the zooid and the exterior. 
Just as in one small portion of the coral the zooids died in the expanded condition, so 
more often, in certain specimens, they die and are preserved with the superficial ecto- 
dermal layer not closed in over the mouth of the calicle, but with the calicle open and 
their retracted tentacles remaining fully exposed to view from above. In Plate 3. 
figs. 10 & 11 both zooids figured are shown in this latter condition. The connexion 
between the superficial ectodermal layer within the calicle and the adjacent vascular 
network of the hydrophyton was not made out. It represents probably merely the largely 
developed ectodermal layer of that part of the network, but the connexion not having 
been seen is not indicated in Plate 3. fig. 10. The superficial layer of the hydrophyton 
being a special development of the ectodermal cells of the vascular network, and the 
interspaces in this network being occupied by calcareous trabeculae, it follows either 
that the tips of the trabeculae at the surface of the hydrophyton must be directly exposed, 
or that the superficial ectodermal cells of the network must close in over them. The 
latter arrangement seems to occur ; and in vertical sections of the decalcified hydrophyton 
numerous spaces left by removed calcareous structures are seen in the superficial ecto- 
dermal layer (see Plate 3. figs. 10 &16), with the ectodermal cells arching over to cover 
them. I should have had no doubt in this matter had I not observed that in the living 
Milleporcc the soft parts of the hydrophyton appear to be retracted below the surface of 
the corallum when the zooids are in their retracted condition. It can, however, hardly 
he the case that any part of the corallum is directly exposed to the water. It is probably 
always covered everywhere by the superficial layer of the ectoderm, which, however, is 
in the recent condition so transparent as to escape observation. The calcareous tissue 
of the corallum must obviously be deposited by the ectoderm, with which alone it is in 
contact. It spreads by extension of the trabeculae at the surface ; and since there it is 
seen to be often in contact only with the cells of the superficial layer, it seems that 
these cells must have the power of producing it. The calcareous network undergoes 
thickening in the deeper parts of the hydrophyton, as must necessarily be the case, 
because of the formation of the tabulae and lines of growth. In these parts no doubt 
the fusiform nucleated cells of the ectoderm are the instruments of the deposition of 
the carbonate of lime. No special calciferous tissue was observed, such as exists in 
Heliopora coerulea. 
Beneath the layer of ectodermal cells in the vascular network of the hydrophyton 
t 2 
