The nervous system is in the form of a network of nerve cells and nerve fibers present in the mesogglia. Many organisms form corals and help form coral reefs (a unique aquatic ecosystem).
Class Scyphozoa
Class: Cubomedusae: Medusoid forms, with bell cubical and margin bent inward
Class: Anthozoa: All marine, solitary or colonial, includes sea anemones, corals and sea pens etc
Subclass: Zoantharia (Hexacorallia)
Many species are solitary and lack any protective covering (sea anemone), some are colonial but never polymorphic (true stony corals or scleractinian corals) iii. Example: Cerianthus inhabit slime-lined vertical tubes in the ocean floor and take their tentacles and oral disk out of the probe at the time of feeding.
Obelia geniculata (Sea fur)
- Habit and Habitat
- Structure
Perisarc forms three to four annuli at the base of the hydrotheca surrounding the polyp. The basal disc of the polyp is continuous with the coenosaic arch of the stem to which it is attached.
Blastostyle: Blastostyle is a zooid which asexually produces numerous medusa buds from its upper end. Blastostyles are produced in the axils of the vertical branches
These are present at the tips of almost all branches of the colony. At the base of the polyp, hydrotheca is produced into a ring-like horizontal platform or shelf.
Medusa: Medusa is a gonozooid which is produced by asexual budding from blastostyle. Mature medusae are present towards the terminal end of the blastostyle and
Nerve cells and fibers are concentrated around the edge of the cell and form two nerve rings, inner and outer. Inner nerve rings around the bell edge are associated with the rhythmic contractions of the medusa.
Epidermis: Epidermis is thin and is made up of many different types of cells which are structurally and functionally differentiated (Fig.4)
Penetrant type: It is the largest type of cnidoblast and is present in Hydra. The network of nerve cells and fibers is dense in the area of the oral disc and tentacles.
Mesogloea: It is very thin in Hydra and Obelia colony except medusa where it is thin only on the subumbrellar surface and very thick on the exumbrellar surface of the medusa
Gonads: Medusa is a sexual zooid as it bears gonads for the sexual reproduction (Fig
When the gonads mature, the epidermis bursts and the sperm or egg cells are released into the water. Digested food is absorbed and circulated throughout the colony by flagellated cells present in the gastrodermis.
Respiration: Exchange of gases takes place through general surface of the body as the colony is present within water. As water also enters through mouth in the gastrodermal
Partially digested food enters the muscle nutrient cells and is digested in the cells. In a jellyfish, the food is captured with a highly extensible mouth and then carried into the gullet.
Reproduction
They also float passively in the water due to water currents and winds blowing over the sea. Glandular cells present in the gastrodermis release digestive juices into the gastrovascular cavity, which break the food into smaller pieces and partially digest it.
Asexual mode of reproduction: Obelia colony grows in size by budding of new horizontal stems (hydrorhiza) and vertical stems (hydrocauli), thereby increasing the
Egg cells can end up in the water or remain in the gonads to be fertilized. Fertilization: Fertilization can occur in water if both eggs and sperm are released into the water or if sperm can fertilize the eggs in the gonads of female jellyfish.
Blastostyles: These are asexual polyps and produce medusae by budding from them
Briefly, metagenesis is the regular alternation of a colonial, asexual, diploid, polypoid generation with a solitary, freely moving, diploid, sexual, medusoid generation. Obelia is one of the very important organisms that show the phenomenon of polymorphism, as it carries three different types of zooids.
Medusae: These are diploid reproductive zooids and reproduce sexually by producing haploid sperms or ova in the water. They lead a free living life and help in dispersal of
Aurelia aurita (Jelly-fish)
- Introduction: The class scyphozoa is represented primarily by the medusoid form, which is more dominant phase in their life history whereas the polypoid stage is restricted
- Habit and Habitat
- Structure
Oral Arms and Mouth: A very short manubrium is present in the center of the submandible, which has a mouth at the tip (Fig. 1b). From the four corners of the mouth come four wrinkled and tender oral hands. Radial canals: The corners of the mouth and the four radial canals arising from them are per-radial in position (Fig. 1b and Fig. 2).
Velarium: There is a thin, delicate, flexible, marginal flap called the velarium, which is a continuation of the surface of the sub-umbrella and carries tentacles and 8 sense organs called.
Velarium: There is a thin, delicate, flexible, marginal flap called velarium which is a continuation of the sub-umbrellar surface and bears tentacles and 8 sensory organs called
Histological structure: The arrangement of body wall layers and cell types are similar to that of Obelia or Hydra
The body is roughly circular in circumference interrupted by eight notches placed at the 4 per-radial and 4 inter-radial positions.
Gastrodermis: The gastrovascular cavity (stomach), radial canals originating from stomach and their extensions into the rhopalia or tentaculocysts, circular canal at the
Locomotion: Locomotion in Aurelia is brought about by the sudden rhythmic contractions of the muscles and gelatinous mesogloea. Mesogloea functions as a skeletal
The mucus-filled food is then swept towards the bell rim, from where it is carried by flagellated grooves in the mouth arms towards the mouth and then to the stomach. Digested food is then circulated through radial channels by the ciliated gastrodermal cells, while undigested food is expelled from the mouth by the exhalation stream of water. Inspiratory flow of water enters the mouth, passes through the narrow esophagus into the stomach and then to the stomach pouches.
The expiratory water flow returns to the stomach pouches via the branching interradial and perradial ducts.
Respiration and Excretion: The highly branched gastrovascular cavity and constant circulation of water brings in oxygen along with food while carbon dioxide and
Reproduction: Asexual mode of reproduction is completely absent in Aurelia
The abdominal spines are only present in the larval stage of Aurelia and are absent in the adult. The presence of abdominal ridges is one of the characteristic features of the scyphozoans, and these are considered a preliminary development of mesenteries present in anthozoans. Ephyrae break away from the strobila one by one as they grew up and begin to swim freely in the water.
All mouth arms are free from each other at the periphery and connected in the form of a disk-like structure towards the middle.
Metamorphosis: The ephyra freely swims in the water feeding on microorganisms
- Alternation of Generation
- Bell margin is notched into 8 lobes bearing 8 rhopalia present at the per-radial and inter
- Pigment cup ocellus and pigment spot ocelli are present
- Gastric ridges bearing gastric filaments present
- Introduction: The presence of polymorphism in cnidarians is one of their characteristic features. It is defined as the occurrence of structurally and functionally
- Class- Hydrozoa: Phenomenon of polymorphism is exhibited generally by the animals of class hydrozoa but a few animals belonging to class anthozoa like Penaatula
The first ephyra larva, which breaks from the parent, bears the original tentacles of the scyhistoma, which later fall off. When food is scarce and the temperature is high, only one ephyra is produced from the scyphistoma larva, this is called monodisc strobilation. But Aurelia does not present a true case of change of generation, for here the medusoid phase develops as a result of metamorphosis of an ephyra developed as one of the several segments of the scyhistoma.
The bell edge is smooth and bears 8 sense organs (statocysts) present at the base of the ad radial tentacles.
Order- Hydroida
Each polyp is present at the terminal end of the branch and has a conical manubrium bearing a mouth, surrounded by two rings of tentacles. The reproductive zooids are called gonophores because they are degenerate medusae that hang in clusters from the base of the gastrozooid (Fig. 4). Tentaculozooids: These are zooids with long blunt ends which are present at the periphery of the colony.
It has a large central gastrozooid, surrounded by 3-4 circles of gonozooids, which are further surrounded by dactylozooids bearing nematocysts, arranged in the periphery of the disc.
Modifications of polyp: Polyps structurally get modified into different types of zooids according to the requirement of an individual, which are described below
Class Anthozoa: Polymorphism is generally absent from anthozoans but one example is given below which shows dimorphism
Small polyps called siphonozooids are present on the dorsal side of the rachis, which lack tentacles and help circulate water in the colony. From the above it is clear that coelenterates have polymorphism and that polymorphism has evolved mainly in colonial animals and also in hydrozoans, except for rare anthozoans. The importance of polymorphism is reflected in the division of labor between different forms and in helping the survival and dispersal of colonial animals.
Mesenteries are septa that grow from the gastroderm of the body wall of the colon and join the pharynx in the upper half of the body.
Primary mesenteries or complete mesenteries: These are those paired mesenteries that are connected to the body wall on one side and to the stomodaeum
- Octocorallians: are polypoid animals which are colonial, bear eight pinnate feather like tentacles, 8 complete mesenteries, have one siphonoglyph and supporting skeleton is
- Hexacorallians: are polypoid animals which are colonial bear either six pairs of mesenteries or in the multiples of six. The number of mesenteries is generally equal to
- Examples of anthozoans showing different arrangement of mesenteries
- 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 remains of the four mesenteries are unpaired and bear muscles directed downwards (Fig. 5a and b). Alcyonium has one of the simplest body shapes and the simplest arrangement of mesenteries found among the anthozoans. The rest of the four pairs of mesenteries consist of one complete (macro-septa) and other incomplete (micro-septa) mesenteries with endocoelic retractor muscles in each pair.
Some of the stages have a special arrangement of mesenteries which is found to be present in some adult anthozoans.
Subclass: Octocorallia or Alcyonacea a. Structure of Octocorallian coral
Heliopora (blue coral): It is light blue in color and is found along with true corals on coral reefs in the Indo-Pacific. It is one of the precious red corals found in the Mediterranean and Japanese seas. Inside the mesogloy is a network of branching tubes called solenia, which are extensions of the coelenteron (Figure 6).
The color of the gorgonians depends on the color of the calcareous spicules and symbiotic zooxanthellae growing in the colony.
Subclass: Hexacorallia (Zoantharia)
Class: Hydrozoa: Some animals of the class hydrozoa like Millepora also contribute towards the formation of coral reefs
They excrete an enormous skeleton and thus contribute significantly to the development of coral reefs.
Coral reefs
Development of coral reefs: The development of coral reefs takes place in four stages
This is called coral zonation, which varies from reef to reef depending on environmental factors (Fig. 13b).
Types and structure of Coral reefs There are three types of coral reefs
The Great Barrier Reef is the contribution of all the different types of coral growth over the years. Atoll: Atolls are coral reefs that are present within seawater hundreds or thousands of kilometers away from the nearest seashore. In some places the reef is so low that the sea water just covers it and the reef is not visible.
Theories that explain the formation of coral reefs: Many scientists have tried to explain the formation of coral reefs and have given their views, which are briefly discussed below.
Theories explaining the formation of coral reefs: Many scientists tried to explain the formation of coral reefs and gave their views which are discussed below in brief
Significance of corals
They help to study the evolution of the animals as fossils of the animals are preserved over the years in the coral reefs. By studying the growth lines on the fossils of some mollusk, it helps to know about the seasonal fluctuations as the thickness of the growth line varies from season to season. A few stony corals due to the presence of small pores are used in surgical procedures as human capillaries can easily pierce through equally sized pores which are useful for connecting two bones together.
Many of the corals are precious stones that are used in the production of jewelry and have aesthetic value.
Coral crisis: There are about 2500 living species of coral many of them are becoming extinct due to change in the environment and global warming which is declining their
These corals are used in bone graft surgery and jaw surgery etc. Finally, corals act as emotional buffers against erosion and storm surges that occur in the sea, helping to prevent tsunami disasters. Coral crisis: There are about 2,500 living species of coral, many of which are becoming extinct due to environmental changes and global warming that are reducing their numbers.
Natural Factors Affecting Coral Growth: Few factors that have a major impact on building coral reefs are briefly given below.
Natural factors affecting coral growth: Few factors which have a great impact on the building of coral reefs are briefly given below
Man made factors: There are several factors which are dependent upon the humans which contribute in destroying or damaging the coral reefs
Human population growth and development: Human population growth is not only responsible for the depletion of natural resources but also causes damage to coral reefs. Increased nutrients can enhance the growth of animals and other boring sponges that outcompete corals to take up space on coral reefs. Commercially they select most of the healthy colonies and sell them at higher rates, thus causing a huge loss to coral reefs.
Marine traffic: Coral reefs are also damaged by fuel oil leaks from large tankers into the water.
Coral Bleaching: Coral bleaching is the whitening of coral colonies due to the loss of symbiotic zooxanthellae from the tissues of polyps. This loss exposes the white
Due to overfishing, an overgrowth of algae can occur, which will not allow the planula larvae to settle and thus coral reefs are unable to settle. Acanthaster planci which protrudes its stomach into the coral and digests the layer of living tissue and causes drastic damage to coral populations. Rules are of no use if people are not properly educated all over the world so that they are ready to protect natural resources and thus coral reefs.
So it is a start to not only protect the islets from disappearance, but also to preserve the coral community.
