SPICULES
Like any other animals, sponges possess a skeleton that gives their bodies shape and structure. All the sponges have a skeleton embedded in the mesenchyme. Skeleton consists of separate spicules or interlacing sponging fibers or both. Skeleton supports and protects the soft body parts of the sponges. Skeleton also serves as the basis of classification of the sponges into various classes like Calcarea, Hexactanellida and Desmospongia.
Structure and types
Spicules are microscopic crystalline structures which gives the sponges their rigidity and form. Spicule consists of spines or rays that radiate from a point. These are secreted by special mesenchymal amoebocytes called scleroblast cells.
On basis of type of deposit on core organic matter:
All kinds of spicules have a core of organic material around which either calcium carbonate or colloidal silica is deposited. Accordingly, spicules are of two types:
Calcareous spicules:
The organic material in this type of spicules is calcium carbonate or calcite. This is the characteristic of the sponges of class Calcarea.
Siliceous spicules:
The organics material in this type of spicules is Colloidal silica or Silicon. These types of spicules are the characteristic of the sponges of class Hexactanellida.
On the basis of size and function:
Spicules can be of large size or small size. Accordingly, spicules can be of two types:
Megascleres:
These are larger spicules constituting main skeleton of sponge body.
Microscleres:
These are the small spicules occurring interstitially.
On the basis of number of axes and rays:
Spicules may occur in several forms like the simple rod form or in the form of forks, anchors, shovels, stars, plumes etc. The spicule forms depend on the presence of number of axes and rays.
Megascleres:
The megascleres are the larger skeletal spicules that constitute the chief supporting framework of the sponge. There are five general types of megasclere spicules, viz., monaxons, tetraxons, triaxons, polyaxons and spheres.
Monaxon: These kinds of spicules are formed by the growth along one axis. They may be straight needle-like or rod like or may be curved. Their ends may be pointed or hooked or knobbed. Monaxons can be both calcareous and siliceous types.
These monaxon spicules are further divided into two kinds,
Monactinal(style)- the growth of the spicule takes place only in one direction.
Diactinal- The growth of the spicule takes place in both the directions.
Tetraxon: These spicules have four rays each pointing in different direction.
Triaenes: one of the four rays is elongated giving the appearance of a crown of 3 rays. Such spicules are called as triaenes.
Calthrops: Sometimes all the rays are equal, when all the rays are equal it is termed as calthrops.
Tetraradiate: When all the four rays persist, it is called as tetraradiate or quadriradiate.
Triradiate: Sometimes one of the rays is lost and then it is known as triradiate. These triradiate rays are characteristic of calcareous sponges.
Amphidisc: If the elongated ray bears a disc at both ends, it is called as amphidisc.
Triaxon: These spicules have three axes that cross one another at right angles to produce six rays. Thus, it is also called hexactinal spicule. These triaxon spicules are characteristic of glass sponges of the class Hexactanellida.
Polyaxon: These are the spicules with several equal rays radiating from a central point. They may be grouped to give star-like appearance. Polyaxon spicules are found along with microscleres.
(v) Spheres:
These are rounded bodies in which growth is concentric around a centre.
Microscleres:
The microscleres are the smaller flesh spicules that occur strewn throughout the mesenchyme. However, they do not form the supporting framework. The microspheres are of two types, viz., spires and asters.
(i) Spires:
Spires are curved in one plane or spirally twisted and exhibits many shapes.
Sigmas: The most common types are the C-shaped forms, called sigmaspires. Spirally twisted sigmas are termed sigmaspires.
Toxas: The bow-shaped ones, or toxas and
Chelas: The chelas with recurved hooks, plates, or flukes at each end.
Isochelas: When two ends are alike, chelas are called isochelas,
Anisochelas: When unlike, anisochelas.
(ii) Asters:
Asters include types with small centres and long rays and large centres and small rays.
Oxyasters: Among the small, centred forms are oxyasters with pointed rays, Strongylaster: with rounded ends.
Tylasters: with knobbed rays. Large-centred forms include spherasters with definite rays.
Sterrasters: with rays reduced to small projections from the spherical surface.
Streptasters: Short spiny microscleric monaxons are known as streptasters, of which the principal sorts are the spirally twisted spirasters, rod shapes or sanaidasters, plesioasters with a few spines from a very short axis,
Amphiasters: with spines at each end. Microscleric forms of diactines are microrhabds, microxeas, and microstrongyles.
Development of Monaxon spicules
The calcareous spicules are secreted by special type of cells called as sclerocytes. These sclerocytes are derived from binucleated mesenchymal scleroblasts. A monaxon spicule or each ray of the triradiate spicule is secreted by a group of two sclerocytes. Among these two sclerocytes one acts as thickener cell and the other acts as the founder cell. The initiation of the formation of the spicule starts with the deposition of a particle of calcium carbonate between the two nuclei of the binucleated mesenchymal cells. This particle grows drawing apart the two nuclei and then two sclerocytes are formed. Now the thickener cell lays down additional layer of calcium carbonate adding to the thickness of the spicule. When the spicule is fully formed, both the cells i.e., thickener cell and the founder cell wander into the mesenchyme. The scleroblast secreting a calcareous spicule is called as calcoblast, while the scleroblast secreting a siliceous spicule is called silicoblast.
Development of triaxon spicules:
Triaxon or triradiate calcareous spicules are secreted by three scleroblasts which come together in triangle and divide in two, each into an inner founder and an outer thickner. Each pair secretes a minute spicule and these three rays are early united into a small triradiate spicule.
Each ray is then completed in the same manner as a monaxon spicule. Later, three rays or spicules unite forming a triaxon or triradiate spicule.
Development of Tetraxon spicules:
In the formation of quadriradiate or tetraxon spicules, the fourth ray is added to forming triradiate spicule by an additional scleroblast. The hexactinal spicules of Hexactinellida arise in the centre of a multinucleate syncytial mass which is probably formed by repeated nuclear division of an original silicoblast.
SPONGIN FIBERS
Structure of spongin
Spongin is an organic, horny, elastic substance which resembles silk in chemical composition. It is a scleroprotein containing sulphur and chemically related to collagen. It is insoluble in water and chemically resistant to protein digesting enzymes. Spongin fibers are fine threads consisting of a soft granular axial core externally surrounded by concentric layers of spongin.
Spongin contains large amounts of iodine. This is the reason that in olden days the bath sponge was used a cure for croup, a throat condition in children resulting from inflammation and partial obstruction of larynx.
• In the class Desmospongia, spongin fiber occurs in various forms.
• It may occur as a cement connecting siliceous spicules.
• It may be found in the form of branching fiber in which siliceous spicules are embedded.
• In Keratosa, spicules are completely absent and spongin alone is formed.
Development of spongin
Spongin fibers are secreted by flask-shaped mesenchyme cells called as spongioblast cells. During the development the spongioblast cells are arranged in the rows and the spongin rods secreted by them are fused with the neighbouring cells to form a long fiber. Later the spongioblasts vacuolated and finally get degenerated after secreting certain amount of spongin.
