Phylum : Porifera

Porifera has been derived from 2 Greek words. Poros – pure and ferre – to bear

Important characters:

1. Their body possesses numerous, minute pores called ostia.
2. They are aquatic, mostly marine and few are fresh water.
3. They are sedentary and sessile except the free swimming, larval form.
4. They are multicellular animals having cellular grade of body organization.
5. They have cell aggregated body plan.
6. They are diploblastic having two body layers – outer ectoderm (also called as pinnacoderm) and inner endoderm (also called as conoderm).
7. They are usually asymmetrical but few are radially symmetrical (sycon).
8. They are acoelomates.
9. They have well-developed canal systems where water enters through ostia into the body, circulates inside the body and goes out through osculum.
10. Water canal system helps in respiration, nutrition, excretion etc.
11. Their body is supported by skeleton called spicules which are made up of Calcium carbonate, Silica or Spongyn fibre.
12. Nutrition is holozoic.
13. Digestion is intacellular.
14. Excretion and respiration takes place by general body surface.
15. Reproduction is sexual as well as asexual. The sexual reproduction occurs by gametic fusion whereas asexual reproduction takes place by external budding as well as internal budding (gemmule formation) and regeneration.
16. T has indirect development with free swimming larvae “amphibulastula”.
17. They are hermaphrodite but fertilization is cross.

On the basis of the chemical nature of spicules types of the skeleton they possess, phylum porifera has been further divided into the following classes:-

Class 1 :Calcarea:-

1. These are exclusively marine and mostly live in shallow seawater.
2. These are solitary or colonial (usually).
3. The body is cylindrical or vase shaped with bristly outer surface.
4. The skeleton is made up of calcareous spicules, which may be simple rod like monaxon, triaxon or tetraxon.
5. E.g. Sycon, Leucosolenia, etc.

Class 2 :Hexactinellida:-

1. All are marine found in deep sea.
2. They are mostly solitary.
3. The animals are attached to the substratum by a root tuft of spicules.
4. The skeleton is made of six rayed siliceous spicules. These spicules are minute, transparent and shining glass – like, hence called “Glass rope Sponges” à they are cup – shaped & shining .
5. E.g. Myalonema, Euplectella, etc.

Class 3: Demospongiae:-

1. They are mostly marine and few are fresh water forms.
2. They are solitary or colonial.
3. The skeleton is made up of siliceous spicules or massive or variable body shape.
4. Spongynfibres or both are absent in some sponges. In oscarella spicules are absent.
5. They are the dominant group of porifera.
6. E.g. Euspongia, Songilla etc.