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In this article we will discuss about Spicules:- 1. Meaning of Spicules 2. Classification of Spicules 3. Development 4. Taxonomic Importance.
Meaning of Spicules:
The spicules or sclerites are definite bodies, having a crystalline appearance and consisting in general of simple spines or of spines radiating from a point.
They have an axis of organic material around which is deposited the inorganic substance, either calcium carbonate or hydrated silica. They present a great variety of shape and as reference to the shape is essential in the description of sponges, a large terminology exists.
Classification of Spicules:
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First, spicules are of two general kinds—megascleres and microscleres. The spicules are further classified according to the number of their axes and rays. Words designating the number of axes end in axons, those referring to the number of rays end in actine or actinal.
1. 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.
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(i) Monaxons:
These are formed by growth in one or both directions along a single axis, which may be straight or curved. When growth has occurred in one direction only, the spicule is called monactinal monaxon or style. Styles are typically rounded (strongylote) at one end and pointed (oxeote) at the other. Styles in which the broad end is knobbed are called tylostyles; those curved with thorny processes are acanthostyles.
Usually the pointed end of styles projects to the exterior. Monaxons that develop by growth in both directions from a central point are named diactinal monaxons, diactines or briefly rhabds. Rhabds pointed at each end are oxeas, lance-headed at each end, tornotes; rounded at the ends, strongyles and knobbed at each end, like a pin head, tylotes.
(ii) Tetraxons:
Tetraxon spicules are also called tetractines and quadriradiates. They consist typically of four rays, not in the same plane, radiating from a common point. The four rays of the tetraxon spicule may be more or less equal, in which case the spicule is called a calthrops.
Generally one ray, rhabdome, is elongated bearing a crown of three smaller rays; such spicules are termed triaenes. By loss of one smaller ray results into a diaene. If the elongated ray bears a disc at both ends, it is called amphidisc. Loss of elongated ray results into a triradiate or triactinal spicule, called a triod characteristic of calcareous sponges.
(iii) Triaxons:
The triaxon or hexactinal spicule consists fundamentally of three axes crossing at right angles, producing six rays extending at right angles from a central point. From this basic type all possible modifications arise by reduction or loss of rays, branching and curving of the rays, and the development of spines, knobs, etc., upon them. The triaxon spicules are characteristic of class Hexactinellida.
(iv) Polyaxons:
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These spicules in which several equal rays radiate from a central point.
(v) Spheres:
These are rounded bodies in which growth is concentric around a centre.
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(vi) Desma:
A special type of megasclere known as desma occur in a number of sponges. A desma consists of an ordinary minute monaxon, triadiate,or tetraxon spicule, termed the crepis, on which layers of silica have been deposited irregularly. Desmas are named from the shape of the crepis, as monocrepid, tricrepid and tetracrepid. They are usually united into a network and such a reticulated skeleton is called lithistid.
2. 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.
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(i) Spires:
Spires are curved in one plane or spirally twisted and exhibits many shapes. The most common types are the C-shaped forms, called sigmas; the bow-shaped ones, or toxas and the chelas with recurved hooks, plates or flukes at each end. When two ends are alike, chelas are called isochelas, when unlike, anisochelas. Spirally twisted sigmas are termed sigmaspires.
(ii) Asters:
Asters include types with small centres and long rays and large centres and small rays. Among the small centred forms are oxyasters with pointed rays, strongylaster with rounded ends and tylasters with knobbed rays. Large-centred forms include spherasters with definite rays and sterrasters with rays reduced to small projections from the spherical surface.
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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, and amphiasters with spines at each end. Microscleric forms of diactines are microrhabds, microxeas, and microstrongyles.
Development of Spicules:
Spicules are secreted by mesenchyme cells, called scleroblasts. Very little is known about the formation of various kinds of spicules. The process is best known for calcareous spicules. On the basis of development, the spicules may be primary which owe their first origin from a single mother cell or scleroblast, or secondary which arise from more than one scleroblast.
(i) Development of monaxon spicules:
In calcareous sponges, a monaxon spicule is secreted within a binucleate sclerobast, probably arising by the incomplete division of an ordinary scleroblast. The calcium carbonate is deposited around an organic axial thread in the cytoplasm between the two nuclei.
As the spicule lengthens, the two nuclei draw apart until the scleroblast divides into two. One cell, the founder is situated at the inner end, the other the thickner at the outer end of the spicule, since monaxon spicules usually project from the body wall.
The spicule is laid down chiefly by the founder which moves slowly inward, establishing the shape and length. The thickner deposits additional layers of calcium carbonate, also moving inward during this process. When the spicule is completed, both cells wander from its inner end into mesogloea, the founder first and the thickner later.
The development of siliceous spicules is poorly known and requires further exploration. It appears that in most cases they are formed completely with one scleroblast called silicoblast.
(ii) 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 on, three rays or spicules unite together forming a triaxon or triradiate spicule.
(iii) Development of other spicules:
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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.
Taxonomic Importance of Spicules:
The main basis of the classification of Phylum Porifera is the skeletal structures found in them.
We have seen that Phylum Porifera has been divided into three classes:
1. Class Calcarea:
Having calcareous spicules.
2. Class Hexactinellida:
Having six-rayed (hexasters) siliceous spicules.
3. Class Demospongiae:
Having siliceous spicules and spongin fibres.
