PORIFERA

ByShrim Gautam •

The term Porifera was coined by Robert Grant. 
Study of poriferans is called Parazoology.


VVI Examples of Phylum Porifera for Entrance Exams of Nepal:
  • Sycon (Urn sponge or Crown sponge),
  • Leucosolenia,
  • Grantia
  • Hyalonema (Glass rope sponge),
  • Euplectella (Venus flower basket)
  • Pheronema (Bowl sponge)
  • Euspongia (Bath sponge),
  • Spongilla (Freshwater sponge)
  • Cliona (Boring or Sulphur sponge)
  • Chalina (Mermaid's gloves)


General characteristics of Phylum Porifera

· First multicellular animals placed under the group Parazoa.
· Aquatic and sessile. But their larvae are free-swimming form.
· Asymmetrical or radially symmetrical.
· Body is porous. The pores are of two types: small minute inhalant pores are called Ostia while large exhalant pore is called Osculum.
· Body is diploblastic. 
The outer layer is called the dermal layer or Pinacoderm. It consists of Pinacocytes and Porocytes. 
The inner layer is called Choanoderm. It consists of Choanocytes or Collared flagellated cells. 
The collar flagellated cells are characteristics of phylum Porifera.
In between these two layers is a gelatinous layer called mesohyl (Mesenchyme). It consists of wandering cells called Amoebocytes and spicules.
· Cellular-level of body organization. No tissue formation.
· Presence of paragastric cavity or spongocoel.
· Nutrition holozoic type. Digestion intracellular.
· Respiration and excretion occur through the general body surface; ammonotelic.
· Presence of water canal system or aquiferous system which is of 3 types: Asconoid type, Leuconoid type and Syconoid type in adult poriferans.


Canal system in sponges

· This system helps in nutrition, respiration, and excretion.
They are of the following four types.

1. Asconoid or Ascon canal system
· It is the simplest type of canal system found in Leucosolenia.
· Spongocoel is internally lined by choanocytes

Route of water circulation
Incoming water - Through Ostia - Spongocoel through Osculum - to outside
Different types of Canal System in Porifera


2. Syconoid or Sycon canal system
· It is found in Sycon and Garantia.
· Radial canals are internally lined by choanocytes

Route of water circulation
Incoming water - Dermal Ostia - Incurrent canal - Prosopyles - Radial canals - Apopyles - Spongocoel - Osculum - to outside
(I -D - I -P - R- A -S - O -T)


3. Leuconoid or leucon canal system
· It is found in adult spongilla and stelleta.
· It is the most complicated type.
· Choanocytes lines the flagellated tubular limited chambers.

Route of water circulation
Incoming water through dermal Ostia - Incurrent canals - Prosopyles - Flagellated chambers - Apopyles - Excurrent canals - larger channels - Osculum -to outside


4. Rhagon type or Rhagonoid
· It is found in larvae of demonspongiae.
· Choanocytes lines the flagellated chamber.
· Development: indirect. The larval stage is known as Amphiblastula (Sycon) or Parenchymula (Leucosolenia).

· Reproduction sexual or asexual. Asexual reproduction occurs either by fragmentation or by the formation of gemmules. Sexual reproduction takes place by the formation of sperms and ova.
· Sponges have great regenerating power due to the presence of totipotent cells called Archeocytes. They are responsible for the development of the sex cells.


Classification of Phylum Porifera

On the basis of the nature of the skeleton or spicules, there are 3 classes:

1. Class Calcarea
· Skeleton consists of calcareous spicules which may be Monaxon, Triaxon, or Tetraxon.
· Canal system: Ascon or Sycon type.
· Exclusively marine and found in shallow water.
· Examples: Sycon (Urn sponge or Crown sponge), Leucosolenia, Garantia, etc.


2. Class Hexactinellida
· Skeleton consists of siliceous spicules which are triaxon i.e. hexaradiate (with 6 rays).
· Canal system: Syconoid or Leuconoid type.
· Exclusively marine and found in the deep sea.
 Examples: Hyalonema (Glass rope sponge), Euplectella (Venus flower basket)



3. Class Demospongiae
· Skeleton consists of sponging fibers or siliceous spicules or both; spicules are monoaxon or tetraxon.
· Canal system: Leuconoid type.
· Marine as well as freshwater.
Examples: Euspongia (Bath sponge), Spongilla (Freshwater sponge)


Leucosolenia

· Leucosolenia belongs to the class Calcarea of the phylum Porifera.
· Leucosolenia is sessile, colonial and marine.
· Leucosolenia is found in shallow water.
· It is an asconoid sponge with the simplest and the most primitive structural plan.
· Body of Leucosolenia is cylindrical and radially symmetrical.
· The body cavity of Leucosolenia is called spongocoel or paragastric cavity.
· The body wall has numerous minute pores called Ostia, which correspond to mouthlets.
· Single large upper opening is called an osculum (exit).
· Porocytes are special cells for the passage of incoming water.
· Outer layer of cells is called pinacoderm made of pinacocytes.
· Inner layer of cells is choanoderm which lines the spongocoel.
· A mesenchyme is found in between the two layers containing free amoebocytes and skeletal elements.


Cell types found in sponges

S.N Types of cell Their shape and functions
1. Pinacocytes Polygonal flat cells
2. Porocytes Cells with pores
3. Choanocytes Collared flagellated cells with digestion, create water current
4. Myocytes Contractile in nature and close the osculum
5. Archeocytes Totipotent undifferentiated cells that give rise to gametes or new cells
6. Collenocytes Secrete collagen fibres
7. Sclerocytes Secrete spicules
8. Sphongioblasts Secrete spongin fibres
9. Trophocytes Feeding cells
10. Thesocytes Store food


· Myocytes form sphincters around the Osculum and Ostia.
· Choanocytes are flagellated collar cells. They maintain water current and ingest the food particles.
· Spicules are embedded in the mesenchyme. Three different types of spicules present in Leucosolenia are monaxons, triradiates and tetraradiates.
· Spicules of Leucosolenia are made up of calcium carbonate, secreted by calcoblasts.
· Leucosolenia has the asconoid type of canal system.

A Colony of Leucosolenia




T.S. of Leucosolenia showing different cells

The path of water current in Leucosolenia is:
· Dermal ostia → gastral ostia → spongocoel → osculum
· Nutrition is holozoic and digestion is intracellular.
· The food is constituted by protozoans, diatoms, bacteria, etc., coming in with water current.
· Choanocytes ingest food particles in the form of food vacuoles, probably by means of pseudopodia.
· Food is partially digested by means of certain enzymes in choanocytes.
· Food vacuole is transferred to amoebocytes and digestion is completed there.
· The chief nitrogenous waste of the sponge is ammonia.
· Leucosolenia is monoecious, i.e. male and female reproductive cells or gametes are formed in the same individual.
· Leucosolenia reproduces asexually by external budding and sexually by syngamy.
· No special gonads, sperms, and ova are developed from archaeocytes.
· Fertilization is internal.
· Cleavage is equal and holoblastic.
· Zygote develops into a hollow blastula called coelo­blastula.
· Further development results in the formation of stereogastrula or parenchymula larva.
· Parenchymula swims freely and gets attached to substratum, finally developing into an adult sessile sponge.
· The corresponding stages in another sponge Scypha (Sycon) are stomoblastula and amphiblastula.
· Leucosolenia has a free-swimming larval stage, but the adult is sedentary.


Taxonomic Summary

Subkingdom – Parazoa
Phylum Porifera – The Sponges
Subphylum Cellularia (all members possess distinct choanocytes.)


A. Class Demospongiae
Cliona – Boring sponge
Chalina – Mermaid gloves (Dead man's fingers)
Spongilla – Freshwater sponge
Euspongia – Bath sponge
Proterion – Neptune's goblet
Hippospongia – Horse sponge
Hircinia – Horny sponge


B. Class Calcarea – The Limy sponges
Scypha – Urn sponge
Subphylum Symplasma (epithelial and "choanocyte" tissues are syncytial.)


C. Class Hexactinellida – The Glass sponges
Pheronema – Bowl sponge
Hyalonema – Glass‑rope sponge
Euplectella – Venus' flower basket



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