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KINGDOM ANIMALIA: PHYLUM PORIFERA AND PHYLUM COELENTERATA

Introduction:

The Animal Kingdom represents the heterotrophic, eukaryotic and multicellular organisms,

that have been grouped under different phyla based on the following features:

1. The Organisation of the body

2. The Symmetry of the body

3. Germ layers

4. Presence or absence of true coelom

5. Presence or absence of the notochord

As per the presence or absence of the notochord, the Kingdom Animalia can be broadly

classified as:

1. Non-Chordata: Animals that do not have the notochord.

2. Chordata: Animals that have the notochord persistent or at some phase of their

life.

The phyla Porifera and Coelenterata represent the non-chordates. These have been

discussed in details in the following heads.

PHYLUM PORIFERA

General Features:

• These are commonly referred to as sponges. These are generally marine animals

• These are asymmetrical with the cellular level of the body organization, as the loose

cells compose their structure.

• Spongin fibres or spicules make up their exoskeleton.

• The embryo is diploblastic, that is, there is an ectoderm and an endoderm. The

mesoderm is absent.

• These are primitive animals.

Water Canal System in Porifera:

The sponges show a characteristic water canal system that functions for the ingestion and

egestion of the food, circulation of the nutrients, exchange of gases, and the removal of

the metabolic waste. It shows the following structural details:

• The body wall is made up of the outermost pinacoderm (made up of pinacocytes)

that are closely packed. These are followed by the Choanoderm (made up of

choanocytes). These are also called the collar cells and are flagellated. These are

responsible for the movement of the water and food particles. These are also

involved in the removal of the waste. There is a non-cellular layer between the

pinacoderm and the choanoderm called the mesenchyme. There are various types

of cells present in the mesenchyme that help in storage of food, developing the

spongin fibres and development of the gametes. These are collectively called the

amoebocytes.

• The body wall is perforated by the minute openings called the ostia. These ostia

open up in the central cavity lined by the choanocytes called the spongocoel.

• The water moves out from the spongocoel via a large opening called the osculum.

Thus, the continuous flow of water ensures the nutrition, digestion, egestion, excretion

and digestion for the sponges. The Water Canal System can show varying degrees of the

compartmentalization as:

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1. Ascon Type Water Canal System: It is the simplest type in which the size of the

spongocoel is larger and there is a single layer of the choanocytes, e.g.,

Leucosolenia. (Fig. 1a).

2. Sycon Type Water Canal System: The size of the spongocoel reduces and

choanocytes occur in more than one layer, e.g., Sycon. (Fig. 1b)

3. Leucon Type Water Canal System: The spongocoel is greatly reduced and there are

many layers of the choanocytes. It is the complex type of system, e.g., Spongilla.

(Fig. 1c)

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Different Classes of Porifera:

These are classified according to the spicules as described below:

Class Calcarea • Large Choanocyte cells.

• Exoskeleton made of calcareous spicules

• Large spongocoel

• E.g. Leucosolenia, Sycon etc.

Class Hexactinellida • Exoskeleton made of siliceous spicules that have

three or six rays.

• Also called glass sponges.

• E.g. Euplectella, Hylonema etc.

Class Desmospongiae • Exoskeleton can be present or absent.

• If present, made of spongin fibres or siliceous

spicules.

• E.g. Euspongia, Spongilla etc.

Digestion in Porifera:

The digestion occurs in the choanocyte cells as they absorb the nutrition from the water

current entering via ostia. Hence, the digestion is intracellular. The digested food is

absorbed and distributed by the amoebocytes.

Reproduction and Development in Porifera:

The sexual reproduction occurs via internal fertilization. The sponges are hermaphroditic,

the male and female sexes are not present in different individuals. The development occurs

through the larval stage, so they show indirect development.

The asexual reproduction occurs by the means of fragmentation, in which the fragments

of the individual animal develops to form the fragments.

PHYLUM COELENTERATA

General Features:

• The phylum is also called the Cnidaria. It includes 9000 known species, both

freshwater and marine.

• They show tissue level of body organization.

• The embryos are diploblastic, with outer ectoderm and inner endoderm.

• The body wall is made up of an outer epidermis and an inner gastrodermis. The

mesoglea is present in between which can be cellular or acellular.

• They show radial body symmetry.

• The body depicts a blind-sac plan in which there is a single opening for the ingestion

and egestion of the food. There is a large gastrovascular cavity present that has

the common opening.

• They show reproduction through both the asexual (budding) and sexual means.

• The development of zygote involves the larval stage, so they show indirect

development.

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Cnidoblasts:

The Coelentrates are characterised by the presence of the specialized stinging cells

called the cnidoblasts. These are present on the tentacles and are used for capturing

the prey and defense mechanism. Each cnidoblast contains a structure called the

nemotocysts that are filled with proteins and phenols called hypnotoxin. These

nematocysts are present with a capsule. At the anterior end of the capsule, there is

coiled, barbed and hollow thread-like structure. (Fig. 2)

Working of Cnidoblasts:

Their working is similar to the bow-and-arrow device. When prey is located around the

tentacles, the coiled thread quickly unfolds and is shot-out like an arrow from the bow.

It coils around the prey and the hypnotoxins are transferred across the hollow region

that paralyses the prey.

Types of Cnidoblats:

According to the function they perform, there are the following types of the

cnidoblasts:

Penetrants or Stenoteles These are used defence and capture of prey

Small Glutinants They are used to attach the tentacles to the

substratum and allow the movement via tentacles.

Large Glutinants They are used to stick t the surface of the body of

prey.

Desmonemes These coil around the prey and are used for

obtaining the food.

Alternation of Generation in Coelenterata:

Most of the animals exhibit two forms of the body, that alternate with each other and

mark the polymorphism or the alternation of generation or the metagenesis. (Fig. 3).

The forms exhibited are as follows:

1. Polyp Form: It is the sessile or fixed body form that reproduces via the asexual

reproduction and gives rise to more of polyp types and the next form of the body.

2. Medusa Form: It is the free-swimming form of the body that develops through the

asexual reproduction in polyp form. It performs sexual reproduction to produce the

zygote which forms larva. The larva settles to develop into the polyp form.

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The alternation of generation is exhibited by polymorphic animals like Aurelia, Obelia

etc. The monomorphic animals like Hydra do not show this alternation.

Classification of Coelenterata

Class Hydrozoa • Solitary or colonial animals, found in both freshwater

and marine water.

• Exhibit radial symmetry.

• Some exhibit metagenesis but the dominant body

form is polyp.

• The mesoglea is acellular.

• E.g. Hydra, Physalia, ObeliaI etc.

Class Scyphozoa • These are solitary and marine animals.

• They show metagenesis and medusa is the dominant

bod form.

• The mesoglea is cellular.

• E.g. Aurelia, Pelagia etc.

Class Anthozoa • They show only the polyp form of the body.

• The mesoglea is cellular.

• These are restricted to marine water only.

• E.g. Tubipora, Xenia etc.

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