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In this article we will discuss about Metridium:- 1. External Structure of Metridium 2. Internal Anatomy of Metridium 3. Histology 4. Feeding and Digestion 5. Muscular System 6. Nervous System 7. Respiration and Excretion 8. Reproduction.
Contents:
- External Structure of Metridium
- Internal Anatomy of Metridium
- Histology of Metridium
- Feeding and Digestion in Metridium
- Muscular System of Metridium
- Nervous System of Metridium
- Respiration and Excretion in Metridium
- Reproduction in Metridium
1. External Structure of Metridium (Sea Anemone):
Metridium has a short, cylindrical body about 8 cm in length. The body is radially symmetrical and divisible into three distinct regions, viz., oral disc, column and pedal or basal disc.
(i) Oral Disc:
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The upper free end is the flat, circular oral disc or peristome having a large oval mouth on a slight elevation. Around the mouth are numerous short, conical and hollow tentacles in five circles or cycles. Generally each cycle has tentacles in multiples of six, the number of tentacles increases with age. Tentacles are very sensitive to chemical stimulation by food juices; tentacles bear numerous nematocysts.
(ii) Column:
The column may be cylindrical throughout, but in some genera, including Metridium, it is differentiated into an upper short, thin-walled capitulum and a lower main thick-walled scapus. On the column are wart-like tubercles.
In some anemones, such as Metridium and Tealia, the upper edge of the scapus forms a prominent fold called collar or parapet which forms a groove or fossae below the capitulum. In the upper part of the scapus is a circular muscle layer called sphincter which can close the margin of the scapus over the capitulum and retracted oral disc. The animal is capable of extreme contraction and the oral disc can be drawn inwards.
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(iii) Pedal Disc:
The pedal or basal disc is expanded and is used for fixing the animal to rocks or shells, it adheres by mucus secretion and by muscles of the basal disc, but the animal is not sedentary because it can creep by gliding motion of the basal disc which puts out a turgid lobe in the direction of movement, while the opposite end contracts, then waves of muscular contractions pass over the basal disc from behind forward so that the rear end advances first, or the muscular contractions may pass from before backward so that the front lobe is pushed out.
The rate of locomotion is about 8 cm per hour. Anemones never have a skeleton of any kind.
2. Internal Anatomy of Metridium (Sea Anemone):
The internal anatomy of Metridium can be studied with the help of its longitudinal and transverse sections (Fig. 34.2 and 34.3 respectively) in the following headings:
(i) Stomodaeum:
The mouth leads downwards into a long stout tube called pharynx or stomodaeum which extends about two-thirds of the length of the column, it is lined with invaginated ectoderm and it hangs in the coelenteron. In the stomodaeum are two longitudinal ciliated grooves called siphonoglyphs (Gr., glyphe – carving), in some genera there is a single siphonoglyph.
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The cilia of siphonoglyphs cause a respiratory current of water to pass downwards. The external surface is not sufficient to meet the respiratory need of the animal, and the current of water passing through the siphonoglyphs aids in respiration. The cilia of the stomodaeum causes a current which removes water and waste through the mouth.
(ii) Gastro Vascular Cavity:
The stomodaeum extends up to about two-third of the gastro vascular cavity. The gastro vascular cavity of Metridium is partitioned by vertical septa called mesenteries into radial chambers.
(iii) Mesenterie:
From the body wall thick longitudinal septa or mesenteries run radially inwards, they divide the coelenteron into compartments. The mesenteries are of two types, the complete mesenteries run from the body wall to the wall of the stomodaeum, the incomplete mesenteries are connected only to the body wall, they extend into the gastro vascular cavity only a part of the way.
The mesenteries lie in pairs, and on one surface each mesentery bears an endodermal retractor muscle running longitudinally, they can cause intense contraction of the animal by which water is ejected through the mouth. Six pairs of mesenteries are complete, they run from the body wall to the stomodaeum.
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These six pairs are called primary mesenteries, two pairs of primaries are joined to the two siphonoglyphs, they are called directives in which the retractor muscles face away from each other, whereas in all the other mesenteries the muscles face each other. Between the adjacent primary mesentery pairs are six pairs of incomplete secondaries which do not reach the stomodaeum.
Between the primary and secondary mesentery pairs are 12 pairs of smaller tertiaries, similarly there may be 24 pairs of still smaller quarternaries. Below the oral disc the primary and secondary mesenteries are pierced by holes called oral stoma or ostia.
They may also be pierced near the body wall by marginal stoma or ostia. These ostia permit water to flow between the inter-mesenteric chambers. The lower edges of mesenteries are fixed to the basal disc, but their free inner edges in the coelenteron bear thick convoluted gastric filaments or septal filaments which are trilobed in section in the upper part.
The upper part of each gastric filament performs the functions of digestion and water-circulation, they have digestive gland cells, ciliated cells, and nematocyst. The lower part of each gastric filament is exclusively digestive, they have gland cells but no ciliated cells, their cells are phagocytic and engulf food particles for intracellular digestion.
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Certain areas of gastric filaments are the main site for excreting waste materials from the body. The lower part of each gastric filament is drawn out into a thin twisted thread called acontium (Gr., akontion = dart), the acontia bear nematocysts which kill the prey. Acontia can also be shot out through the mouth, then they help to overcome small animals for food.
3. Histology of Metridium:
(i) Body wall:
Externally the anemone is covered by an epidermis having elongated epidermis or ectoderm cells of columnar shape. These ectoderm cells are ciliated on the tentacles and oral disc. A few ectoderm cells of the tentacles and oral disc have muscle processes, besides these there are independent muscle fibres in the epidermis.
Between the ectoderm cells are supporting cells, slender sensory nerve cells, mucous gland cells and nematocysts. The nerve cells are numerous in the tentacles, oral disc, and stomodaeum, but they decrease in the column and are again abundant in the basal disc.
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The amount of mesogloea is very large and thick in Anthozoa where it reaches its highest degree of differentiation. It has a gelatinous matrix containing numerous transverse and longitudinal fibres, and there are scattered stellate amoebocytes and connective tissue cells.
The innermost layer of gastro dermis or endodermis is made of columnar epitheliomuscular cells in which the bases of cells are drawn out into muscle fibres. These muscle fibres are circular in the tentacles, oral disc, column and basal disc, but on the mesenteries they form strong retractor muscles which run longitudinally.
Main contraction of the anemone is brought about by these retractors. Between the epitheliomuscular cells are granular gland cells which secrete enzymes, the endoderm also has sensory cells, and nematocyst on mesenteries and acontia.
(ii) Tentacles:
Histologically tentacles consist of epidermis, mesogloea and gastro dermis. Both epidermis and gastro dermis are made up of tall columnar ciliated cells, Mesogloeal layer is extremely thick and contains delicate fibres and scattered cells.
(iii) Mesenteries:
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The mesenteries are composed of two layers of gastro dermis enclosing a central layer of mesogloea.
The mesenteric filaments show a characteristic structure in their upper portion. Each mesenteric filament has a trilobed shape in section. The central ridge known as the cnidoglandular band consists of nematocysts and gland cells. The two lateral ridges consist of all flagellated (ciliated) cells and are known as flagellated band.
(iv) Nematocysts:
Metridium has four kinds of nematocysts, but these nematocysts of Anthozoa are devoid of cnidocil.
1. Spirocyst has a thin capsule containing a long spirally- coiled tube of the even diameter. They are found only on tentacles and oral disc.
2. Basitrichous isorhiza has an oval capsule, there is no butt, the thread has spines only on the base and is open at the tip.
3. Microbasic mastigophore has a rounded capsule, butt is long and bears spines in a spiral, the thread is long and closed at the tip.
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4. Microbasic amastigophore has an oval capsule, butt is short with spines in a spiral, thread is absent. Nematocysts are found in wart-like batteries on the tentacles. They also occur in the epidermis, endodermis, gastric filaments and acontia.
4. Feeding and Digestion of Metridium:
Food consists of animals or pieces of animals of suitable size, the food may be living or dead; it is carnivorous Food is stunned by nematocysts of tentacles, then the tentacles push the food into the mouth, cilia of the mouth sweep the food into the stomodaeum and coelenteron.
The food is held by mesenteries and broken into smaller pieces. A proteose of the nature of trypsin is secreted by the gland cells of the endoderm cells of gastric filaments and acontia, this breaks up the food in an alkaline medium and emulsifies fats.
After this preliminary extracellular digestion the food broth is ingested by phagocytic endoderm cells and intracellular digestion takes place in these cells by proteoses of the type of pepsin, erepsin, and lipase secreted by gland cells.
Many small sea anemones are ciliary feeders, body cilia beat towards the oral disc sweeping up the food, from the oral disc ciliary currents sweep the food particles tentacles bend and transfer the food into the mouth.
5. Muscular System of Metridium:
Muscular system is highly developed, the ectodermal muscles form longitudinal fibres in the tentacles and radial fibres in the oral disc, but the main musculature is endodermal. Endodermal muscles form a circular layer in the mesogloea of tentacles, oral disc, column, stomodaeum, and the basal disc.
The endodermal muscles form highly developed retractor muscles running longitudinally through the mesogloea of the mesenteries. These retractor muscles are the chief means of contraction of the animal. At the junction of the oral disc and column the endodermal circular muscles form a sphincter by which the body is retracted and it covers the oral disc.
6. Nervous System of Metridium:
Nervous system forms two simple nerve nets, one in the entire epidermis and the other in the endoderm of the mesenteries. Each nerve net has sensory nerve cells and their nerve fibres. The epidermal nerve net has ganglion cells in the tentacles, oral disc and stomodaeum. The two nerve nets are connected by fibres, but there is no centralised nervous control and there is a paucity of reflexes.
7. Respiration and Excretion of Metridium:
Both the respiration and excretion are performed by the process of diffusion as the epidermis and gastro dermis remain in constant contact with water.
The beating of the cilia of the gullet and siphonoglyphs set up a regular water current entering through the siphonoglyphs and going out through the gullet. So, the dissolved oxygen in the water is diffused in and carbon dioxide and nitrogenous wastes are diffused out in the water which goes out from the body.
8. Reproduction in Metridium:
Metridium reproduces both asexually and sexually.
(i) Asexual Reproduction:
1. In some genera, such as Sagartia, asexual longitudinal fission takes place. The basal disc elongates and ruptures transversely. The rupture proceeds upwards dividing the column and oral disc longitudinally into two, the torn edges of each half grow together and new mesenteries are formed.
2. Pedal laceration or fragmentation occurs in several genera. Lobes are constricted off from the basal disc, each lobe grows tentacles and mesenteries to form a new anemone.
At times the anemone moves away from its attachment leaving behind lobes of its basal disc and some mesenteries, these regenerate a new anemone at the old site, while the parent develops its lost portions. In anemones formed by pedal laceration there are many irregularities as to the arrangement and number of mesenteries and siphonoglyphs.
3. If an anemone is cut across the column into two, then the lower part regenerates a new oral disc with tentacles, but the upper part usually fails to form a new basal disc, instead it may form a second set of tentacles on its lower aboral surface, thus, showing heteromorphosis.
4. A few instances of budding from column or pedal disc have been reported.
(ii) Sexual Reproduction:
Sexes are separate. Endodermal gonads form thick longitudinal bands on the larger mesenteries lying parallel to gastric filaments. But some anemones are hermaphrodite. Gametes are produced by interstitial cells of the endodermal gonads, but they mature in the mesogloea. In dioecious forms only sperms are expelled from the male into sea water, the females retain their eggs.
Sperms enter the gastro vascular cavity of the female through the mouth and fertilize the eggs. The fertilised egg forms an oval ciliated planula larva which is free-swimming. The planula undergoes metamorphosis to acquire a mouth, stomodaeum, siphon glyphs, and 6 to 24 mesenteries. The embryo sinks and gets fixed by its aboral end and tentacles are formed on the oral disc.
