72 
POPULAE SCIENCE EEVIEW. 
communicated by transverse branches ; they are situate most 
internally and lie so close to the hard framework that as this 
is being developed it leaves grooves in which these canals lie (P) . 
The other series of conduits is composed of vessels of much 
smaller cahbre and which do not pursue any special course, 
but traverse the sarcosome in every possible direction and 
communicate constantly with each other and occasionally with 
the larger series. Their walls are hardly distinguishable, for 
they appear to be merely tunnels through the fleshy tissue of 
the sarcosome ; they communicate with the general cavity of 
the polypites^ body, which the larger channels do not ; and 
both classes contain a whitish fluid known to the coral fishers 
as the milk, but which is really the blood of these creatures, 
and contains both spicules and corpuscles floating through it. 
The polypidom, which is the portion of the zoanthodeme 
most known to readers, is composed of calcareous matter com- 
bined as a tissue with organic matter. Its structure is best 
seen by making a transverse section and submitting it to 
microscopic examination. The observer will then perceive 
that the central portion of the stem is occupied by a homo- 
geneous coloured mass of mineral character, round which there 
appears to be a ribbon-like fold of more highly tinted matter, 
which does not form a circle but some irregularly oval figure ; 
between this and the circumference one sees alternate con- 
centric rings of strongly and faintly tinted material which are 
divided by numerous radiating dark lines of extreme tenuity. 
These various appearances may be thus accounted for : the 
central irregularly fold ribbon-like mass, is the first hard part 
developed by the young egg-polyjpite ; this has had mineral 
matter deposited all round it year after year, but the deposit 
went on with greatest rapidity in the depressions, and hence 
after a while a circular outhne was produced, the several rings 
indicate the successive depositions, and the dark radiating 
lines point to the grooves in which in earlier days the central 
large canals surrounded the stem. 
The foregoing remarks apply to the animal as it is. We 
have now to inquire how is the creature produced. This 
involves an examination of the reproductive system. The 
coral is unisexual, bisexual, and hermaphrodite. Sometimes a 
colony is met with which contains either all males or all females, 
at others we find a zoanthodeme, some of whose branches con- 
tain males, some females ; and it occasionally though rarely 
occurs, that we detect a single polypite, in which the apparatus 
of both sexes are present (fig. 2 B') . In all cases the arrangement 
and structure of the organs are similar. The ova and sperm cells 
are both united to those singular, coiled- thread-like masses 
which we saw attached to the partitions of the general cavity. 
