COELENTEEA — STKOMATOPOEES AND ALCYONAEIANS. 49 
and, by secreting spicules of a horny substance impregnated Gallery X. 
with carbonate of lime, give it greater consistency. The 
common Alcyoniu-m diyitatum (dead man’s fingers) lias such 
a skeleton ; it falls to pieces on the death of the animal, but 
isolated spicules have been found fossil. Sometimes the 
spicules become tightly wedged together and form a compact 
skeleton which cannot be disintegrated ; the precious coral 
{Corallium rubrum) is the hardest skeleton so formed, but 
has only been found fossil in some of the later rocks, and that 
rarely. The organ-pipe coral (Tubqjora) is also well known, 
but is not found fossil. Some of the Gorgonacea form an Table-case 
axis partly liorny and partly calcareous ; and some calcareous 
segments referred to Isis, and a few other forms have been 
found in Tertiary and Cretaceous rocks. A horny axis 
supports the colony of the sea-pen, Pennatula, and in some 
allied forms this axis is partly calcified ; a few such axes 
have been detected in Triassic and later rocks. In the living 
Heliu'pom, the skeleton is formed, not from spicules developed 
in cells, but from lamella; of calcite crystalli,sed in an organic 
matrix produced by the disintegration of ectoderm cells. 
This genus is found fossil back to the Albian Age. With 
the exception of a supposed Triassic Pennatulid, fossils that 
can with certainty be referred to Alcyonaria are confined to 
rocks of Upper Cretaceous and later age; and yet, as was 
the case in Hydrozoa, there are a number of Palaeozoic 
genera that resemble many of the recent forms. 
In the Sub-Chiss ZOANTHARIA, the only living forms 
with a skeleton capable of fossilization are several genera 
differing a good deal in their structure but conveniently 
grouped together as Madreporaria. Several of them might 
be roughly described as sea-anemones with a skeleton. 
This skeleton is quite different from that of the Alcyonaria, 
except perhaps Heliopora. It consists of crystalline car- 
bonate of lime secreted by special ectoderm cells, not within 
the cells but outside them, and outside the whole of the 
ectoderm. The skeleton of the Alcyonaria is internal, that 
of the Zoantharia is external, as is the shell of a snail. 
When the young coral-embryo settles down on the sea- floor, 
it deposits a layer of skeletal substance between its skin and 
the sea-floor, forming a plate, which soon is turned up at the 
edges like a saucer. The soft body of the coral may be 
wholly supported within the saucer, or it may pass beyond 
its rim. In either case the rim is still built up, and at the 
same time lamime of lime stretch out from it to the centre 
