Introduction example of fresh water sponge and skeleton system in sponges.

Introduction:

The fresh -water sponges are commonly known asSpongilla. The freshwater sponge, any of about 20 species of the genus Spongillaknown.The systematic position of Spongilla is below.

Phylum: Porifera

Class: Demospongiae

Sub -order: Monaxinida

Order: Haplosclerina

Genus : Spongilla

Spongilla species are commonly found in clean lake water and slow streams. Freshwater sponges are delicate in structure, growing as encrusting or branching masses. They usually appear greenish because of the algae that live on them. Freshwater sponges may attain in a volume of more than 2,500 cubic centimetres(150 cubic inches). The larva of the spongillafly lives as a parasite on freshwater sponges.

Spongilla attached themselves to rocks and logs and filter the water for various small aquatic organisms such as protozoans, bacteria, and other free- floating pond life. Unlike marine sponges, freshwater sponges are exposed to far adverse and variable environmental conditions, so they have developed gemmules as a mean of dormancy. When exposed to excessively cold or otherwise harsh situations, the sponges from these gemmules, which are highly resistant "bugs" that can live dormant after the mother sponge has died. When condition improved the gemmules "germinate" and a new sponge is born.

Spongilla was used by John Hogg in the 19th century to attempt to justify the fourth kingdom of life.

Spongilla

Ecological notes of freshwater :

• Spongilla is one of the best-known freshwater sponges.
• The colony of spongilla is profusely branched exhibiting various shades of green colour due to the presence of Zoochlorellae a green alga in the tissues.
• The body wall consists of the very thin dermal membrane provided with dermal pores or Ostia and several oscula.
• The canal system is rhagon type.
• Reproduction both asexual (by gemmules) and sexual (by walls of unusual free swimming larva which is characteristics of Spongilla.
• The skeleton consists of siliceous spicules in the form of network,of smooth or spiny large and small oxeas embedded in the sponging.
• Spongilla is a colonial sponge. It is abundantly found in ponds, lakes and slow steam growing on submerged sticks and plants.

Example of freshwater sponge:

Skeletal system in sycon:

Almost all sponges are provided with a skeleton, found embedded in a mesenchymal.This may consist of separate spicules or of interlacing sponging fibres or of both. Skeleton supports and protects the body parts and also serves as the basis of classification of sponges.

Spicules:

The spicules or sclerites are definite bodies, heaving 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. First spicules are two general kinds- 1.Megascleres, 2. Microscleres. The spicules are further classified according to the number of their axes and rays.

A.Megascleres:

These are the larger skeletal spicules that constitute the chief supporting framework of the sponge.There are five types of megasclere spicules.

i) Monoaxons:These are formed by growth in one or both directions along a single axis. When growth has occurred in one direction only, the spicules is called style. Styles are typically rounded at one end and pointed at another end. Monaxons that develop by growth in both direction from a central point are named diactinal monaxons and that develop by growth in the single direction are named monoactinal monaxons.

ii) Tetraxons: They consists of typically of four rays, not in the same plane, radiating from a common point. The four rays of the tetraxons spicules may be more or less equal, rhabdomere , 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 triradiate spicules called a triod characteristics of calcareous sponges.

iii) Triaxons: These consist fundamentally of three axes crossing at right angles, producing six rays extending at right angles from a central point. The triaxons spicules are characterised of class Hexactinellida.

iv) Polyaxons: These are spicules in which several equal rays radiate from a central point.

v) Spheres:These are smaller flesh that occurs strewn throughout the mesenchyme. However, they do not form the supporting framework. These are of two types: spires and asters.

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Spongin:

It is an organic, elastic substance allied to silk but apparently of variable composition.Like nails, hair and feather, it is a scleroprotein containing sulphur and chemically related to collagen, horny protein. It is generally stated to yield leucine and glycine. It is insoluble, chemically inert and resistant to protein digesting enzymes.Spongin contains a large number of iodinereaching 8 to 14 percent in certain tropical species of the Spongiidae and Aplysinidae. Spongin as a skeletal element occurs in two distinct forms. First as a cuticle secretion of a tenacious but elastic cementing substance which glues siliceous spicules together into a more or less definite system of skeletal fibres and secondly in the form of minute elastic fibrillae secreted in the cells and furnishing a tissue which may be compared to the elastic tissue of higher animals.Spongin occurs in various forms in class Demospongiae. It may occur in a cement connecting together siliceous spicules.Spongin fibres are fine threads consisting of a soft granular axial core or medulla, surrounded externally by a concentric layer of sponging.A remarkable property possesses by the spongin fibres of many sponges is that if taking up foreign particles of various kinds into their interior.

Development of spicules and spongin:

Calcareous spicules are secreted by special mesenchymal cells, called splenocytes derived from sclerocytes . A binucleate sideroblast giving the two cells from the rays of spicules.Deposition of calcium carbonate between two nuclei of scleroblast begins the process of its formation. The particles of calcium carbonate grow thickness of spicules increases and the fully formed spicules go to mesenchyme.

Spongin fibres are secreted by mesenchymal cells termed spongioblast. These arranged themselves in rows and the sponging rod secreted by each fused with those of adjacent spongioblast to form the long fibres. The spongioblast becomes vacuolated and degenerate after heaving secreted a certain amount of sponging.

Reference:

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L.Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

http://www.biologydiscussion.com/

https://en.m.wikipedia.org/wiki/Sponge

http://www.britannica.com/animal/freshwater-sponge