An Example of Phylum Cnidaria: Sea Anemone

In this article we will discuss about Sea Anemone:- 1. Habit and Habitat of Sea Anemone 2. Structure of Sea Anemone 3. Nutrition 4. Histology 5. Nervous System 6. Reproduction 7. Development.

Contents:

1. Habit and Habitat of Sea Anemone
2. Structure of Sea Anemone
3. Nutrition in Sea Anemone
4. Histology of Sea Anemone
5. Nervous System of Sea Anemone
6. Reproduction in Sea Anemone
7. Development of Sea Anemone

1. Habit and Habitat of Sea Anemone:

The sea anemones are found abundantly in the sea shores. They are sessile animals and remain attached with rocks, sea-weeds, molluscan shells or any other solid substra­tum. The metagenesis or alternation of generation, so common in other classes of Cnidarians, is completely absent in case of the sea anemones.

They are represented only by hydroid or polyp stage and the medusoid generation is absent. Most of the sea anemo­nes are brightly coloured. When touched the animal contracts its body immediately (Fig. 12.23). The example discussed here is known as Urticina.

2. Structure of Sea Anemone:

Urticina has a cylindrical body which remains attached to the substratum by, its aboral end (Fig. 20.24). The aboral end is closed and forms a broad sole-like base or pedal disc. Rows of adhesive warts are present on the surface of the upright column.

The free end of the body possesses a horizon­tal plate known as the peristome or oral disc, ventral to which lies an elongated slit-­like opening, the mouth. Between the mouth and disc there are eighty short hollow coni­cal tentacles arranged in five circlets. On the body wall small openings known as cinclides are present.

Urticina exhibits both bilateral as well as radial symmetry.

3. Nutrition in Sea Anemone:

Mouth leads into a flattened cylindrical gullet whose terminal edge at each end produces a descending lappet. Two longitu­dinal grooves are present on the stomodaeum known as the siphonoglyphs. Be­tween the gullet and the body wall there are radiating partitions, called the mesenteries or septa (Fig. 12.24B and C).

Mesenteries are of three types:

1. Primary mesenteries:

When the parti­tions are complete. At each end of the gullet two such mesenteries are present. They are called the directive mesenteries.

2. Secondary measenteries:

When the partitions are incomplete and run half way between the gullet and the body wall.

3. Tertiary mesenteries:

Tertiary mesenteries are just small ridges on the inner wall of the body.

The gullet opens into the stomach which gives off a number of inter-mesenteric cham­bers. Inter-mesenteric chambers are in com­munication with one another by two aper­tures on each mesentery, known as the ostia.

The free end of the mesentery below the gullet gives rise to a twisted cord, called mesenteric filament, which is produced into slender threads known as acontia. The acontia are seen to protrude out of the body wall through the cinclides.

4. Histology of Sea Anemone:

The body wall is composed of an outer ectoderm and an inner endoderm and in between them lies a thick and tough mesoglea (Fig. 12.25B). The gullet has a median mesogleal layer with ectoderm on either side. Mesenteries have a supporting layer of mesoglea with endoderm on two sides.

His­tological picture is best seen in the section of the tentacle (Fig. 12.25A). Transverse section of the tentacle reveals that both ectoderm as well as endoderm are made up of tall colum­nar ciliated cells. Mesogleal layer is extremely thick and contains delicate fibres and scat­tered cells.

Elongated cnidoblasts of several types (Fig. 12.26B, 3-5) occur in the ectoderm of the body and in the mesenteric filaments. Gland cells are also abundant in the ectoderm. Mesenteric filament is a trilobed structure with a glandular median lobe and two ciliated lateral lobes. Cnidoblasts are quite abundant in the mesenteric filaments.

Urticina has well-developed muscles which are spindle-shaped nucleated fibres. Mesenteries are provided with longitudinal or retracor muscles, transverse muscles and parietal muscles (Fig. 12.26A).

The longitu­dinal muscles are responsible for the con­traction of the body and tentacles, while the transverse muscles act in a reverse way. The parietal muscles draw the body to the base, where another set of circular muscles is present to act as sphincter.

5. Nervous System of Sea Anemone:

Nerve fibres with nerve cells are found in the ectoderm and in the endoderm. Nervous system is simple and essentially a nerve-net type.

6. Reproduction in Sea Anemone:

Fission or fragmentation may result into the formation of a new polyp. But most common is sexual reproduction. The sexes are separate. The gonads develop in the mesenteries and thus are endodermal in origin.

The spermatozoa are discharged to the exterior through mouth and find their way to reach the ovum and eventually fertilization takes place internally and the zygote is formed. In some Anthozoa both the gam­etes come out of the body and thus fertiliza­tion is external.

7. Development of Sea Anemone:

The zygote, after passing through the routine stages of development, transforms into a typical Cnidarian Planula larva, which after a brief free-swimming existence settles down to metamorphose into an adult. Figure 12.27 illustrates the important features and the plan of lifecycle of a few cnidarians.