Phylum Porifera - Practice Questions & MCQ

• Phylum Porifera contains the simplest invertebrate animals. 
• They are commonly called as ‘sponges’.
• They possess cellular level of organization. Different cells perform the essential functions of ingestion, digestion, excretion, reproduction etc.
• They are sessile, that is, they remain fixed at a place. The larvae are capable of swimming.
• They are filter-feeders.
• Water current allows their nutrition, circulation and excretion.
• They possess a characteristic water canal system.
• Some common examples are:
• Sycon (urn sponge / scypha)
• Euspongia (bath sponge)
• Euplectella ( venus flower basket)
• Cliona (boring sponge; harmful to oysters)
• Leucosolenia (smallest sponge)
• Spongilla (only freshwater sponge)
• Hylanoma (glass rope sponge)

Morphological Features of Porifera

• The name ‘Porifera’ signifies the body made up of pores. 
• Most of the sponges are asymmetric cylindrical in shape with a large central cavity called the spongocoel. 
• There are numerous pores in the body wall called the ostia (sing. ostium) through which water moves in the spongocoel.
• Water leaves the spongocoel through a large pore called osculum.
  

The body wall of sponges can be differentiated into the following layers:

• The outermost layer called outer epithelium or pinacoderm. It is made up of cells called pinacocytes. These cells are scale-like and thin with a prominent nucleus.
•  The innermost layer called the inner epithelium or choanoderm. It is made up of choanocytes or collar cells. These cells trap and digest the food particles coming in through the water current via the ostia.
• Inbetween the two layers, jelly-like layer called mesohyl or mesenchyme is present. It serves to provide skeletal support. Various types of cells are present in the mesohyl namely:

1. Porocytes: These are tubular cells that form the ostia. 
2. Amoebocytes: These are totipotent cells that can form other types of cells. They are named so because of their amoeboid movement throughout the mesohyl. 

• Cells formed by amoebocytes are as follows:
• Trophocytes or nurse cells: provide food to developing cells
• Thesocytes: store food granules
• Collencytes: secrete collagen fibres and form connective tissue
• Chromocytes: pigmented cells
• Gland cells: secrete slimy substance
• Germ cell: produce ova and sperm
• Myocytes: help in closing and opening of osculum
• Scleroblasts: secrete spicules. These can be Calcoblast (calcareous spicules) and Silicoblast (siliceous spicules).
• Spongioblasts:  secrete spongin fibres of mesohyl layer
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