Phylum Porifera & Phylum Coelenterata | Biology Class 11 - NEET

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.

Porifera bearing poresGeneral Characteristics of Phylum Porifera

(a) Habitat

• Poriferans are mostly found in oceans (marine), but there are some that live in freshwater, like the Spongilla. 
• They tend to be sedentary (stay in one place) and are attached to the floor/substratum or a surface.
• Sponges don’t have a fixed place to live and they can be found living in many different places in the ocean, like shallow tropical reefs or deep-sea trenches. 
• They are really good at adapting to different environments. 

Spongilla (Freshwater)

Note: Even though sponges have a lot of abilities, they don't have a brain as we do. This is something they lack compared to other animals. It took millions of years after sponges evolved for an organism with a brain to develop.

(b) Symmetry

• In sponges, symmetry refers to the arrangement of their body structures. Unlike most animals, sponges do not exhibit bilateral symmetry, where their bodies can be divided into equal right and left halves. 
• Instead, sponges display radial symmetry, where their body parts are arranged around a central axis like the spokes of a wheel.
• Radial symmetry is characterized by multiple planes of symmetry passing through the central axis. In sponges, these planes can be either two, three, or more. 
• The most common form of symmetry in sponges is the pentaradial symmetry, where the body is divided into five equal parts along five planes of symmetry.
• The radial symmetry of sponges is related to their lifestyle. As sessile animals, sponges rely on water currents to bring food and oxygen to their cells. The radial symmetry allows for an equal distribution of cells around the central axis, ensuring that all cells have access to the water currents.

(c) Level of Organisation

• Sponges are primitive multicellular animals having a cellular level of organization where division of labour can be seen between the constituent cells.
• Unlike most animals, sponges do not have true tissues or organs, and instead, their body structure is made up of a loose network of specialized cells that work together to perform various functions.
• The cellular level of organization in sponges allows for a high degree of flexibility in terms of their shape and size. They can adapt to different environmental conditions, grow and regenerate lost body parts, and can even fuse together with other sponges to form larger colonies.

(d) Skeleton of Poriferans

• The skeletons of Poriferans, commonly known as sponges, are composed of a variety of structural elements that provide support and protection to the sponge's soft body. The primary components of the sponge skeleton are spicules and spongin fibers.

Spicules in Sponges

• Spicules are small, needle-like structures that are made of various materials, including calcium carbonate, silica, and spongin. They come in a variety of shapes, such as rods, crosses, and stars, and provide rigidity to the sponge's body. Spicules can also help to deter predators from attacking the sponge.
• Spongin fibers, on the other hand, are flexible, protein-based structures that give sponges their spongy texture. They are often found in the body of the sponge as a network of fibers that are interwoven with spicules or other structural elements.
• In addition to spicules and spongin fibers, some sponges also have a more complex skeleton composed of a combination of mineralized structures and organic materials. For example, some deep-sea sponges have a skeleton made of glass fibers, while others have a framework of calcified tubes.
• The complexity and composition of the sponge skeleton can vary widely depending on the species and habitat of the sponge. However, the overall function of the skeleton remains the same: to provide support and protection to the sponge's body. 

(e) Water Canal System in Poriferans

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

Porifera with water canal system

• 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 specialized cells.

(f) Modes of Reproduction and Development of Poriferans

(i) Asexual mode

• In asexual reproduction, a part of the sponge breaks off and grows into a new sponge, which is known as fragmentation. 

• Asexual reproduction in poriferans also takes place by means of budding as shown below:

(ii) 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 fertilization that occurs inside the parental body (female body).
• After fertilization, when the sperm and egg combine, the sponge goes through a process called indirect development. In this process, the fertilized egg develops into a larva that is morphologically different from the adult sponge. 

• Amphiblastula (in Scypha) and parenchymula (in Leucosolenia) are the larval forms.
• The larva then undergoes a process called metamorphosis, where it transforms into the adult sponge form. 
• It is important to note that the process of indirect development is different from direct development seen in many other animals, where the embryo develops directly into the adult form without undergoing a distinct larval stage.

Phylum Coelenterata

The term ‘ Coelenterata ’ comes from ancient Greek ‘hollow’ and ‘intestine’.  Belonging to the animal kingdom, Coelenterata, or more commonly known as the Phylum Coelenterata are the simplest group of animals that fall under the category of invertebrates. They are usually found living at the bottom of the sea, often attached to rocks. 

Characteristics of the Phylum Coelenterata

Usually known to be the simplest of all animal groups, Coelenterates have true tissues and the gastrovascular cavity, which is the characteristic coelenteron. However, these organisms also have a few other characteristic features that make them highly different from other organisms in the animal kingdom, as listed below.

(a) Habitat: Phylum Coelenterata

• These organisms are mostly aquatic. Some live in marine water, for example, Sea Anemones, while some live in freshwater, for example, Hydra. 
  
  Hydra

• They may live in colonies like obelia or may live solitarily like Hydra. 

Colonies of Obelia

(b) Symmetry:  Phylum Coelenterata

• Some Coelenterates are free-floating or are known to swim about freely, like Aurelia. Others are sedentary like Corals. 

Corals

• Their bodies are mostly cylindrical, flat or resemble the shape of a cup. 
• Their bodies can be radially symmetrical with a central gastrovascular cavity or maybe completely bilaterally symmetrical like Sea Anemones. 
  

(c) Level of Organisation:  Phylum Coelenterata

• Cnidarians have a tissue level of organisation, meaning they have distinct tissues that perform specific functions.
• Body organisation of organisms belonging to the Coelenterates are multicellular with tissue grade.
• They have an inner layer of endoderm called the gastrodermis, and outer ectoderm called the epidermis. Their body wall is diploblastic.  

Coelenterata

• Coelenterates are acoelomate animals.
• These organisms have a blind body sac plan. 
• A non-cellular layer of mesoglea is present. This layer is gelatinous in nature. 
• The function of an intestine in Coelenterates is played by an internal hollow cavity called coelenteron, located inside the body. 

(d) Digestion:  Phylum Coelenterata

• Coelenterates have a holozoic form of nutrition.
• Cnidarians have a central gastro-vascular cavity with a single opening, the mouth located on a structure called the hypostome. The mouths of these organisms are encircled with tentacles that have nematocysts. 
• Digestion in cnidarians is both extracellular (in the gastro-vascular cavity) and intracellular (within the cells of the gastro-vascular lining).
• Some cnidarians, such as corals, also have symbiotic relationships with photosynthetic organisms, such as zooxanthellae, which contribute to their nutrition.
• There is no presence of an anus in these organisms; the mouth is responsible for both ingestion and elimination. 
• Coelenterates have tentacles that help them capture their prey, eat and digest it. These tentacles are also used for defence purposes. 

(e) Skeleton:  Phylum Coelenterata

• Some cnidarians, such as corals, possess a calcium carbonate skeleton that provides support and structure to their polyp form.
• There are no circulatory, respiratory or excretory organs in Coelenterates. The outer body surface is responsible for respiratory and excretory functions. 
• These organisms use tentacles and their smooth muscle fibres for locomotion or general movement. Organisms like Corals are fixed on the substratum though. 
• Diffuse nerve cells make up the nervous system in Coelenterates; these organisms, however, do not have a brain.  

(f) Body Forms:  Phylum Coelenterata

• Cnidarians exhibit two main body forms: polyp and medusa.
• Polyps are cylindrical, typically sessile, and attached to a substrate.
• Medusae are umbrella-shaped, free-swimming, and have tentacles for capturing prey.
• Polyps can produce medusae asexually through a process called budding, while medusae form from polyps sexually through gamete release and fertilization.
• Polyps have an exo-skeleton and endo-skeleton. 

• They have interstitial cells due to which the process of regeneration is well developed in Coelenterates. 
• The bodies of these organisms consist of cnidoblasts, which are knob-like structures responsible for the secretion of hypnotoxin chemicals. These chemicals are useful in self-defence or attacking and paralysing the prey.  
• The larva is ciliated and free-swimming. 
• Common examples include Obelia, Hydra, Metridium, Rhizostoma, Xenia, Aurelia and more. 

(g) Mode of Reproduction:  Phylum Coelenterata

• Reproduction is seen in both sexual and asexual modes. While sexual reproduction takes place by the method of fusion of gametes, asexual reproduction takes place by fission and budding. 
• Cnidarians also exhibit a unique phenomenon called alternation of generations or metagenesis. These organisms exhibit the behaviour of polymorphism, which means during their life cycle, they occur in the form of medusa- sexual form and polyp- asexual form. 

Polyp & Medusa behavior of polymorphism

Examples: Examples of cnidarians include Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan), and Meandrina (Brain coral), among others.