Phylum Porifera: General Characteristics, Skeleton, Water Canal System, Reproduction, Examples and Classification

You must be familiar with sponges that are used for various purposes in our everyday lives. Indeed soft know! Have you ever imagined life within a sponge? Nonetheless sponges are non-living entities but surprisingly, we have living sponges too. What are living sponges? Bodies of certain animals are adorned with numerous pores giving them a sponge-like appearance. These are called sponges. These are one of the primitive groups of animals belonging to the phylum Porifera. As the name indicates, Porifera has been derived from two words ‘porus’ meaning pore and ‘ferre’ meaning to bear.

Table of Contents:

• General characteristics of phylum Porifera
• Skeleton of poriferans
• Water canal system in poriferans
• Modes of reproduction and development of poriferans
• Examples of poriferans
• Classification of phylum Porifera
• Practice Problems
• FAQ

General Characteristics of Phylum Porifera

Habitat

Poriferans are usually marine, however some freshwater forms also exist (for example, Spongilla). They are usually sedentary which means they are attached to the floor/substratum.

Symmetry

Sponges have asymmetrical bodies as they cannot be divided into two equal halves through any plane passing through the centre.

Level of Organisation

Sponges have a cellular level of body organisation where division of labour can be seen between the constituent cells.

Skeleton of Poriferans

Spicules or spongin fibres make up the skeleton of sponges. It provides them structural support.

Water Canal System in Poriferans

Sponges possess a water canal system or water transport system for gathering of food, exchange of gases and waste removal. 

Fig: Water canal system in sponges

They possess a body cavity known as spongocoel which is lined by choanocytes or collar cells. Water enters into the spongocoel through minute pores present in the body wall called ostia and leaves the body cavity through the osculum. During the water movement, choanocytes help generate water current and also capture food particles. The food particles are subsequently digested intracellularly by specialised cells.

Types of Canal System in Sponges

• Asconoid canal system: Simplest type of canal system found in Leucosolenia etc.
• Syconoid canal system: More complex than asconoid type, occurring in Sycon etc.
• Leuconoid canal system: Most complex canal system which is common in Spongilla etc.
  • Rhagon type: Leuconoid canal system is derived from a larval stage called rhagon in members of class Demospongiae. Hence, it is named so.

Modes of Reproduction and Development of Poriferans

Asexual mode

Asexual reproduction in poriferans takes place by means of fragmentation as shown below:

Gemmule formation also occurs in sponges.

Sexual Mode

Sponges reproduce sexually by means of gamete formation followed by the fusion of gametes of opposite sexes. Sponges are usually hermaphrodites, i.e., sexes are not separate.

They undergo internal fertilisation that occurs inside the parental body (female body).

Indirect development is observed in poriferans as development of the embryo into adulthood occurs through intervening larval stages which are morphologically different from the adults. Amphiblastula (in Scypha) and parenchymula (in Leucosolenia) are the larval forms.

Examples of Poriferans

• • Scypha (Sycon)
  • Freshwater sponge (Spongilla)

• Bath sponge (Euspongia)

Classification of Phylum Porifera

Phylum Porifera is classified into three different classes based on the nature of the exoskeleton.

Class Calcarea

Sponges having a calcareous exoskeleton characterised by the presence of calcareous spicules are classified under class Calcarea.

Examples include Leucosolenia, Sycon etc.

Class Hexactinellida

Sponges having a siliceous exoskeleton composed of siliceous spicules (made of silica) are classified under class Hexactinellida.

Examples include Euplectella (Venus flower basket), Hyalonema etc.

Class Demospongiae

Sponges having an exoskeleton made up of spongin fibres with or without siliceous spicules belong to the class Demospongiae.

Examples include Spongilla, Cliona, Chalina etc.

Practice Problems of Phylum Porifera

Q1. Which of the following is sedentary or does not show locomotion?

• Spongilla
• Amoeba
• Physalia
• Octopus

Solution: Spongilla belongs to phylum Porifera. It is sessile and sedentary as it is attached to the substratum. 

Hence, the correct option is a.

Q2. Members of class Calcarea and class Hexactinellida differ from each other in:

• Nature of skeleton
• Presence of skeleton
• Presence of tentacles
• Both a and b

Solution: Members of class Calcarea have a skeleton composed of calcareous spicules while siliceous spicules make up the skeleton of members of class Hexactinellida.

Hence, the correct option is a.

Q3. Match the following.

Solution: 

Q4. The current of water flow in the spongocoel is maintained by

• amoeboid cells
• collar cells
• ostia
• osculum

Solution: Collar cells or choanocytes are specialised flagellated cells present in the inner layer of the sponge's body wall. These are distinguishing features of sponges and perform a crucial function in keeping the water flowing through the spongocoel.

Hence, the correct option is b.

FAQs of Phylum Porifera

Question1.- Describe the water canal system in sponges.

Answer. Sponges possess a water canal system which ensures uptake, transport and exit of water through their body. The route of water is 

Ostium (minute pores on the sponge’s body) → Spongocoel (body cavity in sponges) → Osculum (outlet of water)

It helps in gathering of food, exchange of gases and waste removal.

Question2.- Mention the unique characteristics of phylum Porifera.

Answer. The unique characteristics of phylum Porifera are as follows:

• Presence of water canal system
• Presence of ostia and osculum
• Presence of a skeleton composed of spicules, spongin fibres

Question3.- Name the three types of porifera.

Answer. Phylum Porifera is classified into three different classes based on the nature of the exoskeleton:

• Class Calcarea
• Class Hexactinellida
• Class Demospongiae

Question4.- Development of embryos of sponges occurs by ________ means.

Answer. Sponges show indirect development. They have intervening larval stages, morphologically distinct from adults during the development of an embryo into an adult.

Other Related Topics

Other Related Topic Of Biology

• classification of animals based on body plan
• classification of animals based on symmetry
• Phylum Platyhelminthes
• Phylum Ctenophora
• animal kingdom levels of organisation