Porifera: Overview | Zoology Optional Notes for UPSC PDF Download

Sponges are uncomplicated aquatic invertebrate animals that are categorized within the Phylum Porifera, the lowest multicellular members of the Animal Kingdom, comprising approximately 5000 species. These creatures, known as Poriferans, possess small openings called 'Ostia' distributed throughout their bodies. Their sponge-like structures are capable of absorbing and retaining fluids.
Sponges typically remain affixed to the substrate and do not exhibit mobility. Due to this immobility, they were initially mistaken for plants. However, subsequent discoveries regarding their life cycle and feeding mechanisms unequivocally confirmed their classification within the animal kingdom. Continue reading to uncover more fascinating details about the general characteristics of poriferans.

Phylum Porifera Definition

The Phylum Porifera are sedentary, primarily marine, radially symmetrical, or asymmetrical, multicellular organisms without definite organ systems. It is the lowest multicellular animal belonging to the kingdom Animalia, and these are pore bearers or pore bearing species. Earlier, these species were regarded as plants due to their green colour and similarity with algae, but after discovering their life cycle and feeding system, they are included in the animal kingdom.
In the below sections, we will read about the history of Porifera.
Sponge

What is the History of Porifera?

Porifera is a phylum that belongs to the Kingdom Animalia and is commonly known as sponges. The sponge or poriferans, named from Latin porus to “pore” and ferre to “bear”, believed that they are the simplest animals on the earth because they do not have true tissues such as muscles, nerves, and internal organs. So, many scientists who study this animal separated them from other Metazoans to the phylum of the Parazoa branch of Kingdom Animalia.

Characteristic Features of Phylum Porifera

The characteristic features of the Phylum Porifera are as follows:

• The members of this phylum are commonly known as sponges.
• Poriferans bear numerous minute pores known as Ostia on the body wall, which leads into a central cavity called spongocoel or perigastric cavity. The spongocoel opens to the outside by an osculum.
• They are the most primitive multicellular animals that show the cellular level of organization (the cells are not organized into tissues and are arranged as loose aggregates).
• They are mostly marine, and a few are freshwater forms.
• They are often asymmetrical, and the body is irregular, cylindrical, or vase-like.
• The body is supported by needle-like structures called spicules made up of silica or spongin fibres.
• They are diploblastic animals and contain an outer dermal layer of pinacocytes and an inner gastral layer of choanocytes.

Labeled Diagram of Sponge

• The nervous and sensory cells are probably not differentiated.
• Canal System – It is also known as the Aquiferous system, and it is a system of interconnected canals through which water circulates and helps in a number of metabolic activities of a sedentary sponge.

Types of Canal System in Sponges

In sponges, the canal system is of three types and is as follows:

i. Asconoid Canal System: It is the simplest type with thin walls. Spongocoel is large and spacious. Choanocytes form the gastral layer and line the whole spongocoel.
Route of water: This type is seen in Leucosolenia.

ii. Syconoid Canal System – It is a complex type with thick walls. Spongocoel is narrow and choanocytes are restricted in radial canals only. 
Route of water: This type is seen in Grantia.

iii. Leuconoid Canal System – It is a much more complex type with highly folded thick walls. Spongocoel is either reduced or absent. Choanocytes are confined in the flagellated chambers which are formed by the evagination of radial canals.
Route of water: This type is seen in Plakina.

• Digestion is intracellular, respiration and excretion occur through the body wall by diffusion.
• Reproduction – In sponges, reproduction occurs by both asexual and sexual means.

Reproduction in Sponges

Asexual and Sexual Reproduction in Sponges

Asexual Reproduction: This mainly occurs by fragmentation, budding and gemmules.

Sexual Reproduction: Sexual reproduction in sponges occurs when gametes are generated. This type occurs with the help of amoebocytes or archaeocytes or sometimes through choanocytes.

These are hermaphrodites, development is indirect with a larval stage called amphiblastula or parenchymula.

Classification of Phylum Porifera

Phylum Porifera is classified into three classes as follows:

1. Calcispongiae or Calcarea

• These are found in marine, shallow, and other coastal water habitats.
• The skeleton is made up of calcareous spicules.
• The body is cylindrical in structure and exhibits radial symmetry.
• Large Choanocytes are present.
• Small-sized species are present in this class.
• Example: Scypha and Clathrina.

2. Hyalospongiae or Hexactinellida

• These are found in marine and also in deep-sea habitats.
• The skeleton is made up of siliceous spicules.
• The body is cylindrical in structure and exhibits radial symmetry.
• Small choanocytes are present.
• Moderately sized species are present in this class.
• Example: Hyalonema and Euplectella.

3. Demospongiae

• Contains the largest number of sponge species.
• These are found in marine or in freshwater habitats.
• The skeleton is made up of spongin fibres or may be absent.
• The body is cylindrical in structure and exhibits asymmetrical body symmetry.
• Very small choanocytes are present.
• Large-sized species are present in this class.
  Example: Spongilla and Spongia.

Phylum Porifera Examples

Some of the major examples of the phylum Porifera are given below:

Sycon

• The classification of Sycon–
  • Kingdom: Animalia
  • Phylum: Porifera
  • Class: Calcarea
  • Order: Heterocoela
  • Family: Sycettidae
  • Genus: Sycon
• Sycon is a marine form found attached to rocks or corals.
• These have a minute opening called Ostia.
• Their bodies are radially symmetrical or asymmetrical.
• They have the capability to regenerate their lost organs.
• They reproduce by both sexual and asexual reproduction.

Spongilla

• Classification of Spongilla–
  • Kingdom: Animalia
  • Phylum: Porifera
  • Class: Demospongiae
  • Order: Spongillidae
  • Family: Spongillidae
  • Genus: Spongilla
• Spongilla is commonly found in freshwater that is usually colonial and attached to substratum like wood pieces in ponds.
• The exchange of gases and excretion is in the form of diffusion.
• Reproduction is by both sexual and asexual means.

Hyalonema

• Classification of Hyalonema–
  • Kingdom: Animalia
  • Phylum: Porifera
  • Class: Hexactinellida
  • Order: Amphidiscosida
  • Family: Hyalonematidae
  • Genus: Hyalonema
• It is a marine and a free-living sponge.
• Commonly known as glass rope sponge.
• Body consists of a ball-like apical part and a long rope-like stalk.

Economic Importance of Phylum Porifera

Some of the economic importance of the phylum Porifera are as follows:

• The members of the phylum Porifera are used commercially for cleaning or bathing sponges.
• They help to clean up the ocean floor by boring into dead shells and corals releasing chemicals to break them down.
• Sponges have a great capacity to absorb water and hence, these are used in surgical operations for absorbing fluid and blood.
• Sponges play an important role in catching oil leaks.
• It can be used to eliminate stink odour from the refrigerator by sprinkling it a small amount.

Within the Kingdom Animalia, the Phylum Porifera stands as one of the phyla. This particular phylum is distinguished by its members, which are characterized as pore-bearing organisms and commonly referred to as sponges. Sponges are hermaphroditic in nature and are aquatic animals with an asymmetrical body structure, exhibiting a wide range of colors, shapes, and sizes.