Histological Organisation of Cnidarians

In this article we will discuss about the histological organisation of Cnidarians.

1. Ectoderm:

In most cases it is distinctly cellular. Presence of ciliated ectoderm in lar­val cnidarians is usually seen. The cellular ectoderm is mainly composed of the sup­porting cells, called the Epitheliomuscular cells. They are usually cuboidal or columnar in shape. They may be thin and flattened (exumbrellar surface of medusae) or may be slender and elongated (in anemones).

The base of the cell-types possesses contrac­tile extensions. These contractile extensions are absent in some trachylines, in majority of scyphozoa and Actinozoa where the basal end is simply amoeboid. It is observed that distinct fibrils are present in epitheliomus­cular cells of Hydra which render stiffness and elasticity to the body.

The ectoderm of the tentacles and the anterior region of the body of all cnidarians contain numerous characteristic Nematocysts. They are very specialised struc­tures having their origin from the interstitial cells.

The nematocysts are the most remark­able cellular derivatives and are the diagnos­tic feature of cnidarians. The detailed struc­tural organisation of a typical nematocyst has already been described in biology of Hydra. Two primary types of nematocysts are encountered in cnidarians.

(1) Spirocysts:

This type is present in Zoantharia. The capsule is very thin and single-walled. The capsular wall is perme­able to water. The thread tube is of even diameter. It is very long, unarmed and re­mains spirally coiled.

(2) Nematocyst proper:

This type of nematocyst is the characteristic of other cnidarians. The capsule is very thick and is double-walled. The capsular wall is not usually permeable to water. The thread tube may be of variable length and is usually armed. The swollen basal part of the thread tube which is devoid of spines is called the butt. This type is usually divided into sev­eral types, depending upon the nature of the thread tube.

I. The tube is closed at the terminal end.

This includes two types:

(i) Rhopaloneme:

It is present in Diphyes. The tube forms an elongated sac-like structure.

(ii) Desmoneme or volvent:

It is present in Hydra. The thread tube is coiled like the cork-screw.

II. The tube is open at the terminal end and is devoid of butt. This type has certain varieties.

A. Isorhiza or Glutinant:

When the tube is of uniform diameter and found in Hydras, corals, anemones and medusae.

(i) Holotrichous:

When spines are present throughout.

(ii) Atrichous:

When the spines are to­tally absent.

(iii) Basitrichous:

When the spines are restricted at the base.

B. Anisorhiza:

When the type is ex­panded at the base with spines throughout the length. Example is Tubularia.

(i) Homotrichous:

Spines are of same sizes.

(ii) Heterotrichous:

Spines are of unequal sizes.

III. The tube is open at the terminal end and possesses a definite butt.

A. The butt is of same diameter through­out its length.

(i) Microbasic mastigophores:

The tube is continued beyond the butt, but never exceeds more than three times the length of the capsule. This type is present in anemones.

(ii) Microbasic amastigophores:

The tube is short and never extends beyond the butt. This type is present in Sagartia, an anemone.

(iii) Macrobasic mastigophores:

The butt is more than four times the length of capsule. This type is present in Millepora.

(iv) Macrobasic amastigophores:

The butt is very long and the tube does not extend beyond the butt as seen in many anemones.

B. The butt is dilated at the base:

This type includes the stenosteles or penetrant types which are present in Hydra.

C. The butt is dilated at the tip. This type of nematocysts is called as the eurytele.

(i) Homotrichous microbasic:

The butt is small and the spines are of same size. This type is present in Pteroclava.

(ii) Heterotrichous microbasic:

The butt is small and the spines are of unequal size. This type occurs in Eudendrium.

(iii) Telotrichous macrobasic:

The butt is longer with spines at the distal end. This type is also present in Pteroclava.

(iv) Merotrichous macrobasic:

The butt is also longer with spines not at the tip as observed in some tubularian hydroids.

The nematocysts develop inside the inter­stitial cells. The interstitial cells containing the nematocysts are called cnidoblasts or nematoblasts. Different types of nematocysts perform different functions. The nematocysts have specialised regional distribution in the body column of the organism.

Although they are present in abundance in the tentacular and oral regions of the body they actually originate elsewhere. The actual mode of mi­gration is already discussed in the descrip­tion of Hydra. The size of nematocysts varies greatly amongst the different groups. The largest nematocyst is found in Halistemma where the length of the capsule may reach 1-12 mm.

As regards the actual mechanism of discharge of nematocyst thread, nothing definitely can be said. Any stimulus, whether chemical or mechanical, may cause violent expulsion of the nematocyst. Increased os­motic pressure in the capsule helps in dis­charging the thread tube out.

The chemical nature of the substance ejected out from the nematocysts is also not properly known. It is highly toxic and causes paralytic effects on the prey after injection. Biochemical analysis of the extracts from the nematocysts of dif­ferent coelenterates showed that the anaesthising toxins, such as the hypnotoxin, thalassin, congestin, etc., are present which are protein in nature.

Gland Cells:

Gland cells are also quite abundant in the ectoderm. The basal disc region of Hydra is composed entirely of tall basal cells which contain muscular extensions. In the sea- anemones the entire body contains gland cells amongst the epitheliomuscular cells. They help in adhesion of the organism.

Sensory Cells:

Sensory cells in the form of slender, elon­gated bodies with one or two or more sen­sory motile bristles are quite abundant. Ec­todermal nerve cells in the form of bipolar or multipolar ganglionic cells are present in the ectodermal and mesogleal layers.

Another most vital ectodermal cell-type which plays the most significant role in the organisation of Cnidarians, is the Interestitial cell or Subepithelial cell. Like the amoebocytes or archaeocytes of sponges, the interestitial cells in cnidarians are perennial undifferentiated embryonic cells. They help in the production of nematocysts, formation of gametes and in the process of budding and regeneration.

2. Mesoglea:

The consistency of this par­ticular layer in cnidarians varies greatly. In most hydrozoans, cellular elements are ab­sent in it in the polyp stage but in the medusae some fibres of unknown origin are present.

The scyphozoan medusae contain few scattered amoeboid cells and fibres in this intermediate layer. But in actinozoans the mesoglea is highly developed with con­siderable thickness and contains distinct amoeboid cells and connective tissue fibres. Recent studies have shown that mesoglea of Hydra is composed of secretory collagenous element (connective tissue).

3. Endoderm:

The endoderm is largely com­posed of Nutritive-muscular cells. In some polypoid cnidarians, the nutritive-muscular cells contain contractile extensions which run as circular muscles. But such contractile extensions are lacking in medusae.

The en­doderm contains gland cells which secrete gastric juices. The nerve cells and sensory cells are present but in lesser number. Pres­ence of symbiotic forms as Zoochlorellae in green Hydra and Zooxanthellae in some cor­als and anemones is also seen.