A. Primary mesenteries or complete mesenteries: These are those paired mesenteries that are connected to the body wall on one side and to the stomodaeum
5. Examples of anthozoans showing different arrangement of mesenteries
a. Alcyonium: It has all the diagnostic features of octocorallians. The polyps (anthocodia) have 8 pinnate tentacles and 8 mesenteries (Fig. 4). The siphonoglyph is single and the side on which it is present is called ventral (no homology to the ventral surface of higher animals). A pair of mesenteries attached to the siphonoglyph have longitudinal muscles facing towards the endocoel ( also called sulcul mesenteries) while a pair of mesenteries present on the opposite side (i.e. dorsal side in respect to siphonoglyph) bear muscles facing away from each other(towards exocoel and are called
asulcal mesenteries). The dorsal mesenteries are often longer than the others, and are developed earlier in the bud but later in the egg. Rests of the four mesenteries are unpaired and bears muscles which are facing down (Fig. 5a and b). Below pharynx, six of the mesenteries hang freely in the coelenteron and bear unilobed ends while asulcal pair of mesenteries bear cilia and help in circulation of water within a polyp and also throughout the colony for the renewal of food and oxygen. Alcyonium has one of the simplest body forms and simplest type of arrangement of mesenteries found in the anthozoans. Similar type of arrangement of mesenteries is also present in the polyps of Tubipora. Many of the hexacorallians like sea anemone and stony corals pass through a larval stage (Edwardsia stage, Fig. 6) in their development in which they resemble a young alcyonarian polyp in having 8 mesenteries.
b. Edwardsia: It has also total 8 mesenteries like Alcyonium. The Tentacles are more in number as compared to the number of mesenteries. There are two siphonoglyphs, so two pairs of mesenteries are directives and are attached to both the siphonoglyphs. Muscles present on the both pairs of directives do not face each other in contrast to Alcyonium where muscles face each other on one pair of directives (Fig. 6). Rests of the four mesenteries are unpaired and their muscles face downwards like Alcyonium. Edwardsia is an important animal as it resembles the octocorallian structure (have 8 mesenteries) and helps in tracing the origin of hexacorallians from octocorallians.
c. Gonactinia: The arrangement of mesenteries of this anthozoan resembles the arrangement seen in Edwardsia. There is an addition of few mesenteries in this animal e.g. 4 new mesenteries (microsepta) facing 4 already existing unpaired mesenteries (seen in Edwardsia) have started growing (Fig. 7). Another difference is that two pairs of secondary mesenteries (incomplete) have also grown up. With the result this animal has total 8 pairs of mesenteries, out of which two pairs are secondary mesenteries, two pairs are directives and 4 pairs are under completion.
d. Halcampoides: It is a primitive sea anemone having two siphonoglyphs. There are only 6 pairs (12) of primary mesenteries out of which two pairs are directives bearing exocoelic muscles (Fig. 8) and four pairs are bearing endocoelic muscles. These first formed paired and complete mesenteries are called protocnemes. Secondary and tertiary mesenteries were not developed at all in this animal. .
e. Halcampa: It has six pairs of primary mesenteries like Halcampoides. In addition it has 6 pairs of small underdeveloped 6 pairs of secondary mesenteries called microsepta in the region of stomodaeum (Fig. 9). So, in this animal rudiments of secondary mesenteries are reported which are completely formed and are reported in the Adamsia.
f. Adamsia: It has two siphonoglyphs. There are total six pairs of primary mesenteries out of which two pairs are called directives and the longitudinal muscles present on them do not face each other while longitudinal muscles present on rest of the four pairs of mesenteries face each other. 6 pairs of secondary mesenteries are fully developed each having endocoelic retractor muscles (Fig. 10). Even, 6 pairs of tertiary mesenteries are also reported in this animal. Now, if we compare the mesenteries of Adamsia with the Metridium, both of them have similar type of arrangement of mesenteries (Fig. 2a).
g. Haloclava: It is decamerous as it has total 10 pairs of primary/complete mesenteries some of which are formed by the extensions of the micro-septa joining the stomodaeum
(Fig. 11). Both pairs of directives have muscles towards the exocoel while rest of the mesenteries has retractor muscles facing towards the endocoel.
h. Zoanthus: The arrangement of mesenteries is quite different from others and bears either 6 or in the multiples of six. There are total 12 mesenteries out of which 6 are complete mesenteries and 6 are incomplete mesenteries. Out of 6 complete mesenteries, two directives are complete and two directives are incomplete (Fig. 12). Rest of the 4 pairs of mesenteries consists of one complete (macro-septa) and other incomplete (micro- septa) mesenteries with endocoelic retractor muscles in each pair. Such an arrangement of mesenteries is called brachycnemous.
i. Epizoanthus: It has total six pairs of mesenteries out of which, 3 pairs are complete mesenteries (macrosepta) i.e. the ventral directives, 4th and 5th mesenteries on each side starting from the dorsal directives (Fig. 13), while 3 pairs are incomplete mesenteries(1st, 2nd and 3rd pair).
j. Cerianthus (a burrowing anemone): Sand living, elongate, solitary polyps. They secrete a tube of the discharged threads of ptychocysts (special cnidoblasts). Cerianthus has a single siphonoglyph and many complete, unpaired and coupled mesenteries. Their number is not fixed, as it grows, more and more mesenteries are added towards the dorsal growth zone (dorsal interseptal space) and not between already existing couples (Fig. 14).
The two ventral directives attached to siphonoglyph are small. The mesenteries present on either side of the directives are larger and reaches to the aboral end. The mesenteries decrease in size towards the dorsal region where new mesenteries are added. The retractor muscles are absent in all the mesenteries.
k. Antipathes (Black coral): It has six tentacles. Mesenteries may be 4, 6 or 10. There are 10 complete mesenteries having rudimentary retractor muscles (Fig. 15a). Another section shows presence of four directive mesenteries and other two lateral mesenteries which bear gonads for the sexual reproduction. In addition there are 4 secondary mesenteries (Fig. 15b). Mesenteries are without mesenteric muscles. Skeleton is branched, made up of scleroprotein (antipathin), consists of black ectodermal chitinous axis covered with thorns. It is found in deep waters.
l. Peachia: It burrows in the sand when adult but its larval form live as parasites or commensal in the radial canals of scyphomedusae (Fig. 16). It has 6 pairs of complete mesenteries and 4 pairs of incomplete mesenteries.
m. Metridium: As we have seen in fig.2a where it has 6 pairs of complete mesenteries, 6 pairs of secondary mesenteries and 12 pairs of mesenteries which are developed sequentially giving rise to total 24 or 12 pairs of mesenteries.
6. Formation of mesenteries: Formation of mesenteries have been reported in some larval forms of hexacorallians i.e. Actinaria. Mesenteries are not added at random but they are formed in a specific manner. Some of the stages have a particular arrangement of mesenteries which are found to be present in some adult anthozoans. This is evident from the arrangement of mesenteries discussed earlier in various anthozoans.. The formation of mesenteries is explained below in brief (Fig 17a to 17f).
Step 1: Initially considering there is no mesentery within the coelenteron. First of all two mesenteries (1st pair mesenteries) are formed at right angles to the pharynx and divide the gastrovascular cavity into two chambers; one is dorsal and bigger (LC) while
the other is ventral and smaller chambers (SC) giving rise to bilateral symmetry (Fig.
17a).
Step 2: 2nd pair mesenteries started appearing in the bigger chamber (LC) Fig. 17b.
Step 3: 3rd pair of mesenteries begin to appear within the smaller chamber (SC) shown in Fig. 17c.
Step 4: 4th pair of mesenteries are formed within the chamber enclosed by 2nd pair (LC) mesenteries, just opposite to the 3rd pair of mesenteries (Fig. 17d). This type of arrangement is found to be existed in Edwardsia (Fig. 6).
Step 5: Two paired mesenteries grew between the 1st, 2nd and 1st, 3rd. Newly formed mesenteries remain incomplete and their longitudinal muscles face already existing 2nd and 1st pair mesenteries (Fig. 17e). In this way, 8 complete and 4 incomplete mesenteries are formed which are also found to be present within Gonactinia (Fig. 7) and with little modification in Zoanthus (total 12 complete or incomplete mesenteries (Fig. 12).
Step 6: A pair of small mesenteries destined to be incomplete mesenteries start appearing within exocoels (Fig. 17f). In Peachia only four pairs of incomplete mesenteries are found to be present. Later on, more and more mesenteries are added which may be secondary or tertiary mesenteries as are already discussed.
In the examples discussed above, 3rd and 4th pair of mesenteries became the two pairs of directives. It is very clear that secondary and tertiary mesenteries are developed only in the exocoels. The arrangement of longitudinal muscles is also very specific as the muscles present on the directives faced exocoelic chambers in majority of animals, in contrast to other mesenteries where muscles faced endocoelic. The gonads are developed in the mesenteries and the sex cells are lodged in the endoderm. Once the sex cells are mature, they are released in to the coelenteron.
7. Significance and function of mesenteries: The main function of the mesenteries is