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
7. Significance and function of mesenteries: The main function of the mesenteries is to increase the surface area. The free terminal ends of the mesenteries are very long and
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
siphonoglyph is present), or biradial symmetry (when two siphonoglyphs are present) is attained. Bilateral symmetry or biradial symmetry is attained during embryonic development and persists into the adult which is another sign of gradual evolution of higher animals having bilateral or biradial symmetry.
Mesenteric filament
Capitulum
Gonads Complete mesentery
Siphonoglyph Fossa
Tentacles
Mesenteric filament
Limbus Gastrovascular cavity
Pharynx Sphincter Mouth
Longitudinal retractor muscles Parapet
Oral disc
Oral ostium
Marginal ostium Transverse muscle Mesenterial filaments
Acontium Cinclide Parietal muscle Acontia
Epidermis
Basal disc
Fig. 1: Longitudinal section of Metridium showing mesenteries and other structures.
. 2a
Mesenteric filaments Exocoel
Siphonoglyph
Endocoel Mesogloea
Gastrodermis Endocoel
Gonads Retractor muscles
Pharynx
A pair of Tertiary mesenteries
Directive couple A pair of Primary mesenteries Paired secondary mesenteries
Gastrodermis Gastrodermis Epidermis
Epidermis Exocoel
Coelenteron
2b
Fig.2a. Diagrammatic transverse section of Metridium through pharynx showing mesenteries.
2b. Diagrammatic transverse section of Metridium below pharynx showing hanging mesenteries.
Parietal muscles
Gonads
Cnidoglandular lobe Gastroderm Mesogloea is Epidermis
Retractor muscles
3a.
Fig. 3b Fig. 3c
Nematocyst
A core of mesogloea Gland cells
Ciliated cells
Ciliated tract
Gland cells
Nematocyst Mesogloea
Cnidoglandular lobe Cnidoglandular lobe
Fig.3a: T.S. of the body wall of sea anemone showing trilobed cnidoglandular lobe of a single mesentery.
3b. Magnified view of T. S. of a trilobed end of mesentery (upper part). 3c. Magnified view of T.S. of unilobed end of mesentery (lower part).
.
Mesenteries
Soft skeleton due to gastrodermal tubes
Pinnules
Opening of pinnule
Siphonoglyph Mouth
Coelenteron Solenia
Gastrodermal tubes Axial skeleton Tentacles
Fig. 4: Structure of an octocorallian polyp
5a
5b
6 7
Exocoel
Epidermis
Siphonoglyph
Asulcal mesenteries or directives, Retractor muscles facing outwards Retractor
muscles facing outwards
Sulcal mesenteries or directives With retractor muscles facing inwards
Stomodaeum Pharynx
Coelenteron Asulcal mesenteries having ciliated ends
Asulcal mesenteries or directives
Endocoel
Mesogloea
Gastrodermis Unilobed end of mesentery
Primary mesenteries
Exocoel
Endocoel
Secondary mesenteries
Microsepta Directives retractor
muscles facing outwards Siphonoglyphs
Fig. 5a: T.S. Alcyonium through pharynx showing arrangement of mesenteries.
5b: T.S. Alcyonium below pharynx.
Fig.6. Cross section through Edwardsia.
Fig. 7. Cross section through Gonactinia.
Exocoelic retractor muscles
A pair of Primary mesenteries
Exocoel Endocoel
Siphonoglyphs Endocoelic retractor muscles
Fig. 8. Cross-section through pharynx of Halcampoides showing 6 pairs of complete mesenteries.
Siphonoglyphs
Secondary mesenteries Endocoel
Exocoel
A pair of Primary mesenteries
Fig. 9. Cross section through pharynx of Halcampa showing 6 pairs of complete (primary mesenteries while 6 pairs of secondary mesenteries are beginning to appear.
Endocoels Exocoel
Tertiary mesenteries
A pair of primary mesenteries
Secondary mesenteries
Fig. 10. Cross section through pharynx of Adamsia.
Siphonoglyphs
Endocoel Retractor muscles
Exocoels
Fig. 11. Cross section through pharynx of Haloclava showing 10 pairs of primary mesenteries.
Incomplete Dorsal directives
Incomplete mesenteries
Complete mesenteries
Complete ventral directives
Fig. 12. Section of Zoanthus through pharynx showing 6 complete and 6 incomplete mesenteries.
1 1
4 3 2
Incomplete Dorsal directives
2 Incomplete
mesenteries
3
4 Complete
mesenteries
5 5
6
Complete ventral directives
6
Fig. 13. Section of Epizoanthus through pharynx.
Ventral directives
Youngest mesentery Siphonoglyph
All Complete mesenteries
Ventral exocoel, new mesenteries are added here.
Fig. 14. Section through pharynx of Cerianthus.
Siphonoglyph
Incomplete mesentery
Siphonoglyphs
Poorly developed retractor muscles
15b 15a Complete mesenteries
Fig. 15a. Antipathes section with 10 complete mesenteries.
Fig.15b. Antipathes section showing 6 complete mesenteries and 4 incomplete mesenteries.
Secondary mesenteries
Siphonoglyph
Primary mesenteries
Fig. 16. Section of Peachia through pharynx.
a b
LC
SC 1
1
2 LC 2
Dorsal Larger chamber (LC)
LC LC
1 1
SC Ventral Smaller
chamber (SC)
c d
4
2 2 2 2
LC
LC
LC LC LC
1 SC BC 1 1 SC SC 1
Present in Edwardsia
3 3
e f
4
Figs. 17a to 17f. Formation of mesenteries. They are numbered according to the order of their development.
6
Present inPeachia
3 2 4
2
5 5
Incomplete mesenteries 1
1
6 6 6
Present in Gonactinia
and Zoanthus 3
VII Corals and coral reefs 1. Introduction:
Corals are hard structures that are made up of calcium carbonate and are deposited into big stony formations by the anthozoans which are exclusively marine water living. All anthozoans are polypoid animals without any medusoid form at any stage of the development. The polyps produce sex cells, fertilization gives rise to formation of zygote developing into planula larva, which directly form new polyp. These anthozoans may remain singly or form big colonies and majority of them secrete hard calcareous structures around them starting from their basal disc. Anthozoans bear cilia on different regions of the body e.g. the cilia present around the mouth beat towards outer side and thus clean the oral end.
Colonial and fixed polyps show a great degree of development by secreting enormous structures called coral reefs which play an important role in maintaining aquatic ecosystem. Coral reefs are submarine ecosystems which can be compared with the rain forests as they also host diverse flora and fauna. The reef building corals require warm shallow waters i.e. above 200C. They are therefore limited to the Indo-Pacific, Central Western Pacific, and the Caribbean regions north to Bermuda, while other corals live at moderate depths throughout the world.
In addition to different types of the coral forming anthozoans, there are variety of other organisms belonging to different groups which also contribute significantly in the formation of big stony structures over the years called coral reefs. Different anthozoans and hydrozoans which contribute to the formation of coral reefs are briefly discussed below.
2. Class: Anthozoa: A majority of the coral forming animals belongs to class anthozoa and structurally resembles sea anemones except that they do not have siphonoglyph. A coral polyp resembles the basic structure of the sea anemone.
A. Subclass: Octocorallia or Alcyonacea