Skeletal Elements Found in Sponges | Phylum Porifera

The following points highlight the two main skeletal elements found in sponges. The types are: 1. Spongin 2. Spicules.

Skeletal Element: Type # 1. Spongin:

The spongin comprises of fi­bres of spongin and they resemble silk in appearance. These are proteinaceous secre­tion of the spongioblasts and form a net­work. In most cases, the spongin fibres are composed of protein, termed spongin-B. In some cases the fibres are composed of colla­gen, called the spongin-A.

Skeletal Element: Type # 2. Spicules:

The spicules or sclerites have definite bodies consisting of spine or spines radiating from a point. Each spicule consists of an axis of organic material around which inorganic substances are deposited either in the form of calcium carbonate or hydrated silica. They exhibit a great variety of form and shape. The spicules are broadly divided into two principal types depending on size.

These are:

(i) Megascleres or the large skeletal spicules and

(ii) Microscleres or the small spicules.

The megascleres actually form the chief skeletal framework of the sponges. The microscleres are present throughout the mesenchyme. But such division of the spicules on the basis of size, is not regarded to be accurate. They are best classified into different types according to the number of their axes and rays (Fig. 11.13).

The types of spicules recorded below are mainly based on the above criteria:

Monaxon type:

They are formed by the growth in one or both directions along a single axis. The axis may be straight or may be curved.

The monaxon type of spicules is of two kinds:

(i) Monaetinal monaxon, when the growth occurs in one direction and

(ii) Diactinal monaxon, when growth occurs by proliferation in both directions from a cen­tral point.

Tetraxon type:

This type of spicule con­sists typically of four rays radiating from a central point. Subsequently by loss of ray or rays, they may secondarily become three, two or one-rayed. There are several varieties of tetraxon spicules.

These are:

(i) Calthrops:

When the four rays are almost equal.

(ii) Triaenes:

When one ray is elongated (rhabdome) and other three are short (cladome).

(iii) Diaenes:

When one of three cladi is lost.

(iv) Triradiate:

A special type of triaene where the rhabdome is lost.

Triaxon type:

This type of spicule is fun­damentally composed of three axes which cross at right angles and produce six rays (hexactinal) arising from a central point.

Polyaxons:

When several equal rays ra­diate from a central part, the spicules are called polyaxon type.

The polyaxons are divided into two broad categories:

(a) With small centres and long rays and

(b) Large centres with small rays.

The first category of spicules is subdivided into:

(i) Oxyasters (pointed rays),

(ii) Strongylasters (rounded ends), and

(iii) Tylasters (knobbed rays).

The second category is subdivided into:

(i) Spheraster (with definite rays), and

(ii) Sterrasters (with reduced rays).

Spheres:

The spicules under this category are rounded bodies where growth occurs in concentric manner around a centre.

Desma:

This type consists of minute monaxon or triradiate tetraxon spicule, called crepis on which layers of silica have been deposited irregularly. The desmas are classi­fied into four types depending on the shape of the crepis.

These are:

(i) Monocrepid,

(ii) Tricrepid,

(iii) Tetracrepid, and

(iv) Lithistid desmas.

The first three types have monaxon, triradiate and tetraxon crepis while the fourth variety is formed by the union of the first three varieties in the form of a network.

Development of spicules:

Spicules are the secretory products of the Scleroblasts. The origin of calcareous spicule is clearly known. The formation of monaxonic calcareous spi­cule begins as minute deposition of calcium carbonate in the interior of a binucleate scleroblast laid down around an organic axial thread. Spicule begins to be deposited be­tween the two nuclei which ultimately sepa­rate the cell into two.

Of the two cells, one is known as the founder cell and the other cell is designated as the thickener cell, which deposits additional layers of calcium carbon­ate. In the formation of triradiate spicule three scleroblasts participate and act more or less in the same way. Figure 11.13 (A-F) shows the formation of monaxon and triradiate spicules. As regards the development of sili­ceous spicule details are not clearly known.

Taxonomic significance of spicules in sponges:

1. Existence of spicules in sponges helps the taxonomists greatly in arranging the diverse species of sponges into regular and definite orders.

2. Classification of sponges is largely based on the type of spicules they possess.

3. It is interesting to note that in porifera evolution has taken place in the mesenchyme and not in the epithelial layer.