Phylum Porifera are the lowest multicellular animals belonging to the kingdom Animalia.
The word “Porifera” mainly refers to the pore bearers or pore bearing species. Based on the embryological studies, sponges are proved as animals and are classified into a separate Phylum in animals.
This phylum includes about 5000 species. Poriferans are pore-bearing first multicellular animals. The pores are known as Ostia.
The poriferans have a spongy appearance and are therefore called sponges. They are attached to the substratum and do not move. They have the ability to absorb and withhold fluids.
They were initially regarded as plants due to their green colour and their symbiotic relationship with algae. Later, their life cycle and feeding system were discovered, and they were included in the animal kingdom.
Characteristics of Phylum Porifera
Some of the important characteristics of phylum Porifera are mentioned below.
The cells of Poriferans are loosely organized.
They are mostly found in marine water. Only a few are found in freshwater.
They are either radially symmetrical or asymmetrical.
Their body is usually cylindrical.
The scleroblast secretes spicules while spongin fibres are secreted by spongioblasts.
They have no organs in their body.
They depict cellular grade of organization.
The body comprises numerous pores known as Ostia and osculum.
The central cavity is called spongocoel or atrium which opens to the outside through the osculum.
They reproduce asexually by budding, and fragmentation.
The nutrition is holozoic.
They have neurosensory cells but are devoid of any specific nervous system.
They have the power to regenerate the lost parts.
The development is indirect and the cleavage is holoblastic.
The exchange of respiratory gases and nitrogenous wastes occurs by the process of diffusion.
Classification of Phylum Porifera
Phylum Porifera is classified into three classes:
They are found in marine, shallow, and coastal water.
Their skeleton is composed of calcareous spicules made of calcium carbonate.
The body is cylindrical and exhibits radial symmetry.
The body organization is asconoid, syconoid, or leuconoid.
Eg., Clathrina, Scypha
They are found in marine and the deep sea.
The skeleton is made up of six-rayed siliceous spicules.
The body is cylindrical in shape and exhibit radial symmetry.
The canal system is Sycon or Leucon.
Eg., Euplectella, Hyalonema
They are found in marine or freshwater.
The body is asymmetrical and cylindrical in shape.
The canal system is a leuconoid type.
The skeleton comprises spongin fibres, siliceous spicules, which are monoaxon and triaxon.
Eg: Spongia, Spongilla, etc.
Some of the common Porifera examples are:
These are solitary or colonial marine sponges found in shallow waters attached to the rocks. The body is cylindrical in shape with numerous spores. The radial canal is made up of flagellated cells. Water enters the body through Ostia and reaches the radial canals by prosopyles. These species undergo both sexual and asexual mode of reproduction.
These are also known as glass rope sponges found in marine water. The body is round or oval with twisted root tufts. Small amphidiscs are present in the skeleton.
They are also known as Boring Sponges found in coral skeletons, mollusc shells, other calcareous objects. They are green, purple, or light yellow in colour. The canal system is the characteristic of the leuconoid type of sponges, and they reproduce asexually and sexually
These are also known as Venus flower basket and are found in deep waters. The body is cylindrical, long and curved fastened in the mud at the bottom of the sea. The canal system is simple synconoid type. The skeleton consists of siliceous spicules fused at the tips forming a three-dimensional network with parietal gaps.
They are largely found in ponds, streams, lakes growing on submerged plants and sticks. The body wall consists of a thin dermis provided with pores called Ostia. They possess a rhagon type canal system. They reproduce sexually as well as asexually.
Also Read: Sexual Reproduction
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Frequently Asked Questions
What are the characteristics of phylum Porifera?
The phylum has the following distinguishing characteristics:
- These are pore-bearing multicellular animals.
- The body has no organs.
- They exhibit holozoic nutrition.
- The body is radially symmetrical.
- They can regenerate their lost parts.
Give a few examples of Poriferans.
Where are the sponges found?
Sponges are found in shallow water and deep seas, but are always found attached to the floor of the sea. They can be found at a depth of more than 8000 metres.
What is the mode of nutrition of Poriferans?
Poriferans exhibit holozoic nutrition. They filter the tiny, floating organic particles and planktons that they feed on, hence called filter-feeders. They collect the food in specialized cells called choanocytes which are transported throughout the body by amoebocytes.
Why are Poriferans confused to be plants instead of animals?
Poriferans are attached to the seafloor and cannot move from one place to the other. Since they share this characteristic with plants, they are often confused to be plants instead of animals.
How are sponges important?
Since sponges are attached to the sea bed, they act as a habitat for several commercially important species, thereby maintaining the biodiversity of the sea and supporting the food web.
Why are sponges considered to be animals?
Sponges do not have chlorophyll and cannot prepare their own food. They capture different organisms for nutrition. Most of the sponges that reproduce sexually produce sperms and eggs. That is why they are considered to be animals and not plants.
Name the different types of sponges.
Sponges are of three types:
The classification of Porifera is based on?
The classification of Porifera into Calcarea, Hexactinellids and Desmospongiae is based on the presence of their exoskeleton.