General characteristics, classification and examples of Porifera

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General characteristics of Phylum Porifera:


1. They are aquatic, mostly marine and few are freshwater inhabitants.

2. They are sessile
 and sedentary.

3. They are either asymmetrical or radially symmetrical.

4. They are multicellular organisms.

5. They have cellular grade of body organisation.

6. The whole body in them is perforated with numerous, minute inhalant pores called ostia through which water enters their body cavity, hence the phylum is called Porifera.

7. They have a broad opening at their apex called oscula through which the water comes out of their body cavity.

8. The outer epithelial layer of their body is called pinacoderm which is composed of flat, polygonal cells called pinacocytes.

9. The inner epithelial layer of their body is called choanoderm which contains microvillous collard, flagellated choanocytes.



General characteristics, classification and examples of Porifera

10. They have a gelatinous, non-living layer which contains amoeboid cell and supportive skeletal element, called mesenchyme which is present between pinacoderm and choanoderm.

11. They have long hollow interior cavity called spongocoel which is lined by choanocytes.

12. Their body skeleton has siliceous spicules or collagenous fibres also called as spongin fibres.

13. They doesn’t contain any definite organs or organ systems.

14. They doesn’t have any mouth and gut and so their digestion occurs intracellularly.

15. Muscles and nerve cells are absent in them.

16. They exhibit the power of regeneration.

17. They are mostly hermaphrodite.

18. Asexual reproduction in them occurs by budding and gemmules whereas sexual reproduction occurs by the formation of gametes.

19. Their fertilisation in them is internal.

20. They have complete and radial cleavage.

Example: Sycon, Spongilla, Grantia, Euspongia.


Classification:

They are classified into 3 different classes, as follows:


CLASS-1

Calcarea or Calcispongiae

1. Commonly they are called as calcareous sponges.

2. Their body size is very small.

3. They are distributed worldwide.

4. Shape of their body is cylindrical or vas like.

5. Their skeleton is composed mainly of calcium carbonate.

6. They generally have monaxon and tetraxon type of spicules.

7. Canal system in them is either sycon, ascon or leucon type.

8. Asexual reproduction in them occurs by budding and branching. Whereas, sexual reproduction is indirect.

9. They are mostly marine inhabitants.

10. Osculum in them is provided with oscular fringe.

Example: Sycon, Grantia.

 

CLASS – 2

Hexactinellida or Triaxonida or Hyalospongiae


1. Commonly they are called as glass sponges.

2. Their body size is large.

3. They are distributed worldwide.

4. Their body shape is curved, urn or vac like.

5. They have simple canal system (either syconoid or leuconoid type).

6. They have siliceous type of body skeleton.

7. Their choanocytes are restricted to flagellated chamber.

8. They are mostly marine inhabitants.

9. Cellular dermal epithelium is absent in them.

10. Megascleres (skeletal spicules) and microscleres (flesh spicules) are easily distinguishable in them.

Example: Euplectella, Hyalonema


CLASS-3Desmospongiae


1. Commonly they are called as people sponges.

2. They are mostly marine inhabitants, but some may be found in fresh water.

3. They have highly organised and complex body structure.

4. Their body shape is rounded, oval, funnel, or curved and may or may not be branched.

5. Skeleton in them is either absent or siliceous.

6. They exhibit rhagon type of canal system.

7. Reproduction in them occurs by both asexual and sexual means.

8. Freshwater species of this class bear contractile vacuole.

Example: Spongia, Euspongia, Thongilla

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