The polyp is a fixed sac that has one opening, functioning
as a mouth and anus, and surrounded by one or several crowns of hollow tentacles
ending, or not, in a swelling, the acrosphere. The mouth is prolonged by
a short œsophagus, the stomodeum, which terminates in the gastrovascular
cavity or cœlenteron, itself prolonged by a straight canal to the extremity
of each tentacle. The stomodeum consists of an ectodermic pharynx and gastric
separations oriented radially, alternating with the tentacles. The upper
part of the animal carrying the tentacles is called the oral disc and the
lower part above the skeleton is the basal or aboral disc. The oral disc
is joined to the basal disc by the mural column. (Figure 2).
The internal free edge of the oral disc has mesenteries which are prolonged
laterally to the mural column. The free internal edge of the mesenteries
often has cnido-glandular filaments known as mesenteric filaments, which
ribbon and roll round the lower part. These filaments can protrude from
the mouth or from the temporary openings of the mural column, openings
which persist some time after the filaments retract. The mesenteries have
muscular fibrils and play an important part in the contraction and extension
of the polyps; they also take part in digestion and enclose the reproductive
organs.
Figure 2.
Schematic representation of a colonial type polyp.
The tentacles are outgrowths of the oral disc. There are as many tentacles
as interradii, which are extensions of the gastrovascular cavity divided
by the mesenteries. The tentacles contain numerous cnidocytes (stinging
cells). They are tactile and take part in defence and nutrition. They can
be expanded or contracted, at night or during the day, according to the
species.
In any coral colony, the polyps are all from an intra-
or extra-calicinale division, in other words an internal division of the
polyp or polyp bud of the existing polyp. The polyps are joined together
by a connecting tissue called cœnosarc (Figure 3) from which the actual
skeleton originates, between the skeletal elements of the polypierites,
the cœnosteum. The cœnosarc is actually a prolongation of the
oral and aboral epithelia which join up to delimit a space corresponding
to the prolongation of the cœlenteron of each polyp, the cœnosarcal
space. Cœnosarcal spaces are rarely simple, and are often divided into
canals by the soft external partitions, the external mesenteries.
Figure 3.
Schematic representation of connective tissue, the cœnosarc.
This tissue links several polyps. It allows for the prolongation and inter-communication
of gastrovascular cavities, the coelenterons.
The oral tissue covers most of the zooxanthellae. In the
coral Stylophora pistillata, the species chosen for this work, the zooxanthellae
are distributed according to a network, the spinules, ornamenting the skeleton.
Figure 4 represents part of a polyp seen through a confocal microscope.
The sample is exposed to light with wavelengths of 568 nm, the chlorophyll
of the zooxanthellae, excited by this wavelength, fluoresces at 590 ±
15 nm. Each white zone indicates the presence of a spinule. Zooxanthellae
are excluded around the spinules due to the thinness of the oral tissue
in this area.
Figure 4.
Observation with a confocal microscope of a portion of a living Stylophora
pistillata polyp.
A part of the mouth of the polyp is on the right of the photo.
Chlorophyll under a wavelength of 568 nm fluoresces at
590 ± 15 nm.
