Q.5. Write short notes on the Structure of body wall of sycon. (2007)
Related Questions -
Q. Draw a longitudinal section of sycon and describe the structure and function of different types of cells found in it. (2011)
Ans. Structure of Body Wall of Sycon: -
It contains two types of cellular layers the pinacoderm and the chonaoderm.
(1) PINACODERM: -
It is made up of two layers -
(i) Exopinacoderm: - Covering the entire body surface except the dermal ostia and osculum.
(ii) Endopinacoderm: - It includes the epithelial lining of the incurrent canals and spongocoel.
The pinacoderm is composed of large, flattened polygonal cells the pinacocytes. Pinacocytes are highly contractile and they can greatly increase or reduce the surface area of the sponge body.
In the lining of the incurrent canals some of the pinacocytes are modified to form tubular cells the porocytes connecting the incurrent canals with the radial canals through their intracellular channels the prosopyles.The porocytes are thin walled cells opens at both ends end with the nucleus present in the periphery cytoplasm.Pinacocytes surrounding the osculum outer or dermal ostia and inner ostia or apopyles are elongated and contractile and acts as muscle cells called the myocytes.They form the sphineters around them to regulate these openings.
(2) CHOANODERM: -
It constitutes the gastral epithelium and is formed of flagellated collar cells or choanocytes. These cells are oval or rounded and arranged in a loose layer upon the mesenchyme. Each cell contains a single nucleus, one or more contractile vacuoles, blepharoplast, rhizoplast and a single basal granule or kinetosome from which originates a long whip like flagellum.
(3) MESENCHYME: -
Between the pinacoderm and the choanoderm lies the interconnecting gelatinous matrix the mesenchyme or mesohyl .It is supposed to be secreted by the pinacoderm.It contains a variety of amoeba like cells the amoebocytes which are modified archaeocytes that migrate from the outer cellular layer and carry on a variety of function essential to the life of the sponge.A few types of ameobocytes are as follows -
(i) Archaeocytes: -
These are undifferentiated embryonic amoebocytes having blunt pseudopodia and large nucleus with clear nucleolus .They can produce all other types of cells needed by the sponge.They also give rise to the sex cells i.e. ova and sperms .They play an important role in regeneration .Such cells are termed as totipotent as they give rise to any other kind of cells within an animal.
(ii) Collencytes: -
Most of the amoebocytes have branching pseudopodia often united into a syncytial network.These are coolencytes.
(iii) Chromocytes: -
These are pigmented amoebocytes with lobose pseudopodia.
(iv) Thesocytes: -
These have lobose pseudopodia and are filled with food reserves thus acting as storage cells.
(v) Myocytes: -
These are contractile amoebocytes around ostia.These regulate the amount of water entering into the body through ostia.
(vi) Scleroblast: -
These manufacture the spicules and according to the nature of the product are known as calcoblast,silicoblast and spheroblasts.
(vii) Gland Cells: -
These secrete an adhesive substance and provide a means of attachment for the animal.
(viii) Germ Cells: -
The ova and sperms of sponges differentiate from amoebocytes though in some cases they have been stated to be modified choanocytes.
Q.6. Write short notes on the sponge gemmule. (2006, 08)
Other Related Question -
Q. Write short note on Gemmule formation. (2016)
Ans. Sponge Gemmule: -
All fresh water and some marine sponges have a regular mode of asexual reproduction by internal buds called gemmules (gr. Gemma-buds) which eventually detach and develop into new individuals.These enable the sponges to tide over unfavorable conditions,such as excessive cold or draught.
In fresh water sponges formation of gemmukees takes place by aggregation of archaeocytes rich in food reserves.A columnar layer of amoebocytes encircling the central mass of archeaocytes secretes a thick hard chitinous inner and a thin outer membrane .Meantime scleroblasts of the sponge secretes silecious spicules,monaxons or amphidiscs,which are laid radially between the two membranes.Later the columnar cells trophocytes and the scleroblasts depart.
A full grown gemmule is a small hard ball consisting of an internal mass of food-laden archaeocytes surrounded by a resistant chitinous covering or capsule made of two membranes and usually pierced at one end by the micropyle.The protective envelope may be strengthened with silecious amphidiscs picules or by monaxon spicules.
In autumn the fresh water sponges suffering from cold and food scarcity ,die and disintegrate leaving behind a large number of gemmules which remain inert throughout the winter.As the body of the parent decays the gemmules are set free and these either sink to the bottom or simply float away.In april or may when conditions again become suitable the gemmules begin to hatch.Their living contents escape through the micropyles and develop into new sponges.These produce ova and spermatozoa and give rise to a summer generation of sponges which in turn die away in autumn leaving behind the gemmules.The life history of such sponges thus illustrates alternation of generation.
Q.7. Write short notes on the spicules of sponges. (2007)
Other Related Questions -
Q. Write short note on types of spicules found in porifera. (2017)
Ans. Spicules of Sponges: -
Spicules are crystalline structures consisting of spines or rays that radiate from a point.These are secreted by special amoebocytes called scleroblasts. All kinds of spicules have a core of organic material around which is deposited either calcium carbonate or silicon.Accordingly the spicules are of two types -
(i) Calcareous which are characteristic of the class calcarea and
(ii) Sileceous characteristic of class hexactinellida.
According to size the larger spicules constituting the main skeleton are called megasclers and the smaller spicules are called microscleres.Further spicules may occur in several forms they may be simple rod like or may take the forms of forks,anchors, etc.These forms depend upon the number of axes and rays present.Accordingly they can be divided into the following types -
(I) MONAXON: -
These are formed by growth along one axis.These may be straight needle like or rod like or curved.Their ends may be pointed,knobbed or hooked.They are of two kinds -
(i) Monactinal-in which growth takes place in one direction only.
(ii) Diactinal in which growth occurs in both directions.
Monaxons are both calcareous and siliceous types.
(II) TETRAXON: -
These spicules are with four rays each pointing in a different direction.Usually one of the four rays is elongated giving the appearance of a crown of 3 rays.Such spicules are termed triaenes. However when all the rays are equal the spicule is termed the calthrops. When all the four rays persist the tetraxon is also termed as tetraradiate or quadriradiate. However when one of the four rays is lost the spicule becomes triradiate which is characteristic of calcareous sponges.If the elongated ray bears a disc at both ends it is called an amphidisc.
(III) TRIAXON: -
These spicules are with typically three axes that cross one another at right angles to produce six rays.It is thus hexactinal.Triaxons are characteristic of glass sponges only.
(IV) POLYAXON: -
These are spicules with several equal rays radiating from a central point.They may be grouped to give bur or star like appearance.Polyaxons are common among microscleres.
DEVELOPMENT OF SPICULES: -
Calcareous spicules are secreted by special cells called sclerocytes derived from scleroblasts. A monaxon spicule or each ray of triradiate spicule is secretes by a group of two sclerocytes one acting as a thickener cell and the other as the founder cell. A binucleate scleroblast is believed to give rise to these cells.Formation of spicule begins as a deposition of a particle of calcium carbonate between two nuclei. The particle grows drawing apart first the two nuclei and then the two slerocytes. Thickener cell lays down additional layers of calcium carbonate adding to the thickness of spicule. When spicule is fully formed both the cells wander into mesenchyme.
Scleroblast secreting a calcareous spicule is called calcoblast while that producing a siliceous monaxon is called silicoblast.