Animal Kingdom: Phylum Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata, Practice Problems and FAQs

We live in a beautiful world. In our surroundings, we can see a diverse range of animals including birds, insects, reptiles, amphibians etc. All animals have their own specialised characteristic features. Can you name some animals from your area? Now you understand that all the animals have different names and they are placed in different groups. Have you ever thought why the animals are different from one another and how they are grouped in different categories?

Fig: Kingdom Animalia

Animalia is a broad Kingdom that is classified on the basis of different characteristic features, such as levels of organisation, symmetry, germ layer organisation, coelom, segmentation, body plan, presence notochord etc. By using these many features as a base, animals are classified into various phyla.

Do you know how many phyla there are? There are 11 phyla in the Animal Kingdom. In every phylum, related animals are placed. Now let’s take a look at all these phyla in detail in this article.

Table of contents

• Kingdom Animalia
• Phylum Porifera
• Phylum Coelenterata
• Phylum Ctenophora
• Phylum Platyhelminthes
• Phylum Aschelminthes
• Phylum Annelida
• Phylum Arthropoda
• Phylum Mollusca
• Phylum Echinodermata
• Phylum Hemichordata
• Phylum Chordata
• Practice Problems
• FAQs

Kingdom Animalia

All animals are included in the Kingdom Animalia. It is the largest Kingdom among all the five Kingdoms. Members of the animalia are eukaryotic and mostly multicellular. However, unlike plants, they lack a cell wall and chlorophyll. Therefore, members of the Animal Kingdom show a heterotrophic mode of nutrition. They are consumers. Approximately, around 7.8 million species of animals are present on the planet Earth.

Fig: Kingdom Animalia

Phyla

Animals are classified into eleven different phyla based on different features as follows:

• Porifera
• Coelenterata (Cnidaria)
• Ctenophora
• Platyhelminthes
• Aschelminthes
• Annelida
• Arthropoda
• Mollusca
• Echinodermata
• Hemichordata
• Chordata

Basis of classification

Animals are classified into several phyla based on the following features:

• Classification of organisms based on the number of cells
• Levels of organisation
• Symmetry
• Germ layer organisation
• Coelom
• Formation of coelom
• Segmentation
• Body plan
• Types of digestive system
• Types of digestion
• Open and closed circulatory system
• Presence of notochord

Levels of organisation

According to the number of cells present, organisms are classified into two types such as unicellular and multicellular organisms.

Protozoa

Unicellular organisms are those in which all functions are performed by a single cell. Examples include the phylum Protozoa.

Fig: Paramoecium - Unicellular

Metazoa

Multicellular organisms or metazoans are those organisms whose body is made up of many cells. Therefore, all the body functions are performed at tissue, organ, and organ system levels in them. Examples include phylum Porifera to Chordata.

GIF: Human lice - Multicellular

Levels of organisation

According to levels of organisation, organisms are classified into four types as follows:

• Protoplasmic level
• Cellular level
• Tissue level
• Organ level
• Organ system level

Fig: Levels of organisation

Protoplasmic level of organisation

It is the lowest level of organisation and is commonly seen in protozoans. Protoplasm consists of cell organelles, nucleus and cytoplasm. Cytoplasm is defined as the complex, semifluid, translucent material that makes up a live cell's inner matter and is made up of proteins, lipids, and other molecules suspended in water. Here the body of the animal consists of a single cell which performs all the vital activities of the body. Examples include Paramoecium.

Fig: Paramoecium - protoplasmic level

Cellular level of organisation

The body structure of these organisms are composed of cells that are loosely arranged. These cells are responsible for performing different functions but the cells are not organised into tissues and organs. Between the cells, there is a clear division of labor. The members of the phylum Porifera possess a cellular level of organisation.

Fig: Sponges (Phylum Porifera) - Cellular level

Tissue level of organisation

The body of the organism is multicellular here. The specialised cells that perform similar functions are organised into tissues. The members of phylum Coelenterata (Cnidaria) and Ctenophora show tissue level of organisation.

Fig: Jellyfish (Phylum Cnidaria) - Tissue level

Organ level of organisation

Organs are formed when tissue performing similar functions aggregates and they perform respective physiological functions. The members of phylum Platyhelminthes (flatworms) show organ level of organisation.

Fig: Phylum Platyhelminthes - Organ level

Organ system level of organisation

In this type the body possess tissues, organs and organ systems. The organ system is made up of the organs necessary to carry out a specific function. The members of phylum Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, and Chordata show organ system level of organisation.

Fig: Humans (Phylum Chordata) - Organ system level

Symmetry

In terms of size, shape, and relative position, symmetry is the distribution of body parts around an axis or on either side of a line of division. It is further classified into three types as follows:

• Radial symmetry
• Bilateral symmetry
• Asymmetric

Radial symmetry

In this symmetry the body of an animal can be divided into two equal halves by cutting it in any plane passing through the central axis. The animals showing radial symmetry are called radiata. These animals possess an oral end and an aboral end. It is commonly seen in phylum Ctenophora, Coelenterata and Echinodermata.

Fig: Radial symmetry in starfish

Bilateral symmetry

In this type of symmetry, the body can be divided into two equal halves only through one plane (the median sagittal plane). The animals that possess bilateral symmetry are called bilateria. These animals possess an anterior and posterior end. The side of the body that is kept forward during locomotion is called the anterior side and the other is called the posterior side. The surface of the body facing the substratum is called the belly or ventral surface. The opposite side is called the back or dorsal surface. It is commonly seen in phylum Chordata, Aschelminthes, Platyhelminthes etc.

Fig: Bilateral symmetry in goat

Asymmetric

Body cannot be divided into two equal halves in any plane in this model. Therefore, the body has an irregular shape. Phylum Porifera shows this model. Examples include sponges.

Fig: Asymmetry in sponges

Germ layer organisation

The different layers of cells in an embryo formed in the gastrula stage is known as the germ layer. It aids in the development of all tissues and organs. There are three germ layers, ectoderm, mesoderm, and endoderm. Based on the number of germ layers present, organisms are classified into two categories as follows:

• Diploblastic
• Triploblastic

Diploblastic

The organisms possess two germ layers, external ectoderm, and an internal endoderm are called diploblastic. An undifferentiated layer called mesoglea is present between ectoderm and endoderm here. It is present in the phylum Coelenterata and Ctenophora. A common example of a diploblastic animal is Hydra.

Fig: The two germinal layers in Hydra

Triploblastic

The triploblastic organisms possess three germ layers, external ectoderm, middle mesoderm, and an internal endoderm. The embryos of all animal phyla from Platyhelminthes to Chordata possess this model. Humans have triploblastic organisation.

Fig: Three germinal layers in humans

Coelom

Coelom is the body cavity present between the alimentary canal and the body wall. It is lined by mesoderm or coelomic epithelium on all sides and is only present in triploblastic animals. Coelom provides protection to the internal organs from accidental shock. It allows changes in the shape and size of the internal organs. Depending on whether there is a cavity between the body wall and the alimentary canal, the coelom is categorised into three types as follows:

• Acoelom
• Pseudocoelom
• True coelom

Acoelom

Acoelom is a condition in which the organism lacks a body cavity or coelom. The organisms are known as acoelomates. It is shown by flatworms or Platyhelminthes, which are bilaterally symmetrical and triploblastic.

Fig: Acoelomate

Pseudocoelom

In this type of condition, between the ectoderm and the endoderm, mesoderm is located in scattered pouches. Mesoderm does not line the body cavity. Examples include roundworms or Aschelminthes which are bilaterally symmetrical and triploblastic.

Fig: Pseudocoelomate

True coelom

These organisms are known as coelomates because they have a true body cavity between the alimentary canal and the body wall. The cavity is lined by the mesoderm from all sides. Examples include humans that are symmetrical and triploblastic. The coelomate phyla include Annelida, Arthropoda, Mollusca, Echinodermata, and Chordata.

Fig: Coelomate

Types of coelom formation

Based on the method of formation of coelom, it is of two types as follows:

• Schizocoelom
• Enterocoelom

Schizocoelom

It develops as a split in the mesoderm sheet. It is found in the phylum Annelida, Arthropoda and Mollusca.

Enterocoelom

Here in this model the mesoderm arises from the wall of the embryonic gut as outgrowths or pouches. These pouches pinch off and enlarge until they squeeze out the blastocoel. It is seen in the phylum Echinodermata and Chordata.

Segmentation

Segmentation is described as the divisibility of the body into similar constituent units. On the basis of segmentation, animals are divided into three categories as follows:

• Absence of segmentation
• Pseudo metamerism
• Metamerism

Absence of segmentation

In this type, the body is not divided into segments. The members of the phylum Porifera and Coelenterata are included in this category.

Fig: Non-segmented body of sponges

Pseudo metamerism

The body is segmented only externally here and the internal segmentation does not form. The members of the phylum Platyhelminthes (Tapeworm) are included in this category.

Fig: Pseudometamerism in tapeworm

Metamerism

The body is divided into segments, with some organs repeating in succession here. For example, the segments contain serial repetitions of excretory organs (nephridia) in earthworms. The members of the phylum Annelida possess this model.

Fig: Metameric segmentation in earthworm

Body plan

The term ‘body plan’ refers to the collection of physical characteristics shared by all members of a given phylum. There are three types of body plan as follows:

• Cell aggregate plan
• Blind sac plan
• Tube within a tube plan

Cell aggregate plan

This type of body plan includes cells that are arranged as loose cell aggregates. Tissues and organs are absent in the organism. This type of body plan is seen in the phylum Porifera (sponges).

Fig: Cell aggregate body plan

Blind sac plan

In this model the body is like a sac with a single cavity or digestive tract having a single outlet or mouth. The one external hole called the blastopore that functions as both the anus and the mouth in an incomplete digestive system. This type of body plan is seen in the phylum Coelenterata, Ctenophora, and Platyhelminthes.

Fig: Blind sac body plan

Tube within a tube plan

In this model the body consists of two tubes, outer body wall and inner digestive tract. The digestive tract or the alimentary canal runs through the body from the mouth to the anus in this model. Ingestion of food occurs through the mouth and the egestion of the undigested waste materials takes place through the anus. The outer tube consists of skin, shells, horns etc. This appears to be a tube inside of a tube with cavities that are filled with fluid in between the two. This type of body plan is seen in Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, and Chordata.

Fig: Tube within a tube body plan

Complete and incomplete digestive system

The digestive system is made up of various organs that work together to digest food materials, absorb nutrients and eject undigested materials. There are two distinct digestive system types:

• Complete digestive system
• Incomplete digestive system

Complete digestive system

A complete digestive system is one that contains two different openings, one for ingestion of food (mouth) and the other for egestion of waste materials (anus). The digestive system of humans is a complete type.

Fig: Complete digestive system in humans

Incomplete digestive system

A digestive system is said to be incomplete if it only has one opening and serves as both the mouth and the anus. The digestive system in Platyhelminthes is an incomplete one.

Fig: Incomplete digestive system in flatworm

Types of digestion

Digestion is of two types as follows:

• Intracellular digestion
• Extracellular digestion

Open and closed circulatory system

A circulatory system is a group of organs that aids in the movement of different metabolic byproducts throughout an animal body. The circulatory system is of two types as follows:

• Open circulatory system
• Closed circulatory system

Open circulatory system

In an open circulatory system, blood immediately bathes the cells and tissues as it passes through the sinuses and body cavities. The circulatory system in phylum Arthropoda and Mollusca is open type. The body cavity of these animals remains filled with blood always.

Fig: Open circulatory system in arthropods

Closed circulatory system

In this type of circulatory system, blood travels through blood vessels and capillaries. It is seen in phylum Annelida and Chordata.

Fig: Closed circulatory system in humans

Notochord

Notochord is a rod-like structure present on the posterior surface of an embryo. It is present only during embryonic development and is derived from mesoderm.

Fig: Notochord during embryonic development

Animals are categorised into two types based on the presence or absence of notochord as follows:

• Chordates
• Non-chordates

Chordates

Animals that possess notochord during the embryonic development are known as chordates. Examples include humans.

Fig: Chordata

Non-chordates

Animals that lack a notochord during embryonic development are known as non-chordates. Examples include sponges.

Phylum Porifera

Porifera includes organisms with pores. The name is derived from the two Greek words Porous means pore and pherein means bearing. These are commonly known as sponges.

Characteristic features of Porifera

The characteristic features of poriferans are as follows:

• They are sedentary, multicellular organisms with a hard outer skeleton.
• They have a porous body.
• They possess a canal system created by pores on the body that aids in the movement of substances.
• The body of poriferans is not differentiated into head and tail.
• They do not have well-defined organs or organ systems.
• They live in marine habitats.
• They are asymmetrical.
• They possess choanocytes, pinacocytes and amoebocytes.

Classification of phylum Porifera

Based on their exoskeleton, phylum Porifera is classified into three different classes as follows:

• Class Calcarea: The exoskeleton of these sponges is composed of calcium carbonate and is calcareous. Examples include Scypha (Sycon).

Fig: Scypha

• Class Hexactinellida or Hyalospongiae: These sponges have an exoskeleton composed of silica and is siliceous. Examples include Euplectella (the Venus’s flower basket).
• Class Demospongiae: An exoskeleton consisting of spongin fibers surrounds these sponges. Examples include Spongilla (the freshwater sponge).

Fig: Spongilla

Common examples of Porifera

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

Phylum Coelenterata or Cnidaria

The Greek word ‘kilos’ means hollow-bellied from where the word ‘Coelenterata’ originated. They are also called Cinidaria. The Greek word ‘knide’ means ‘nettle or sting cells’.

Characteristic features of Coelenterata

Some of the characteristic features are listed below:

• The members of Coelenterata have a hollow body cavity.
• Their body is differentiated into two ends.
• They are aquatic organisms and either sessile or free-living.
• They are radially symmetrical which means their body is divided into two equal halves in any plane.
• They have a tissue level of organisation.
• They are diploblastic organisms, in which cells are arranged in two embryonic layers.
• They have two body forms, polyp, and medusa.
• They show alternation of regeneration.
• Polyp are sessile and reproduce by asesual method. They give rise to medusa. Medusa are free swimming and reproduce by sexual method.

Classification of phylum Coelenterata

Based on the dominance of medusoid or polyploid phase, phylum Cnidaria is classified into three different classes as follows:

• Class Hydrozoa: Either a polyp-only body form or a polyp-and-medusa body form exists. Examples include Hydra, Obelia (the sea fur) and Physalia (Portuguese man of war).
• Class Scyphozoa: Their body structure exists only in medusa form. Examples include Aurelia (jellyfish).
• Class Anthozoa or Actinozoa: Their body structure exists only in polyp form. Examples include Adamsia (sea anemone), Pennatula (sea pen).

Common examples of Cnidaria

• Portuguese man-of-war (Physalia)
• Brain coral (Meandrina)
• Sea anemone (Adamsia)
• Sea-fan (Gorgonia)
• Sea-pen (Pennatula)

Fig: Examples of Cnidaria

Phylum Ctenophora

The name ‘Ctenophora’ comes from the Greek words ‘ktene’ means ‘comb’ and ‘phors’ means ‘bearing.’ Sea walnuts and comb jellies are other names for ctenophores.

Characteristic features of the phylum Ctenophora

The common characteristic features are listed below:

• The members of phylum Ctenophora are exclusively marine.
• They are radially symmetrical.
• They have a tissue level of organisation, which means cells performing similar functions are grouped together to form tissue.
• They are diploblastic organisms and have two embryonic layers.
• They possess eight median comb plates which are formed by the union of the cilia.
• They possess nerve cells.
• Life cycles possess cydippid larvae.
• They are bisexual.

Classification of Ctenophora

Based on the presence or absence of tentacles, they are classified into two classes:

• Class Tentaculata: From the little stomodaeum, which is an oral cavity in the embryo stage, they have tentacles. Examples include Pleurobrachia and Ctenoplana.
• Class Nuda: They have a large mouth and stomodaeum but no tentacles. Examples include Beroe.

Examples of Ctenophora

• Pleurobrachia
• Ctenoplana

GIF: Pleurobrachia GIF: Ctenoplana

Phylum Platyhelminthes

The term Platyhelminthes is derived from the Greek words ‘platys’, which means broad or flat, and ‘helmin’, which means worm. Members of Platyhelminthes are hence also known as flatworms.

Characteristics features of flatworms

The characteristic features of the flatworms are listed below:

• They have a dorso-ventrally flattened body.
• Their body structure is complex and well-differentiated.
• They are primarily endoparasites with few of them are free-living.
• They are bilaterally symmetrical, which means their body can be divided into two equal halves in any plane.
• They have an organ level of organisation.
• They are triploblastic and possess three germ layers, ectoderm, mesoderm, and endoderm.
• They are acoelomates which means the cavity between the body wall and the gut wall is absent.
• They are hermaphrodite.
• The lifecycle shows larval stages. For example miracidium, sporocyst, cercaria, and metacercariae larval stages present in liver fluke. Cysticercus is present in tapeworms.

Classification of Platyhelminthes

Based on the mode of living, they are classified into three classes as follows:

• Class Turbellaria: This class of organisms mostly includes free-living creatures. Examples include Planaria.
• Class Trematoda: Ectoparasitic or endoparasitic organisms are included in this class of Platyhelminthes. They are commonly called flukes. Examples include Schistosoma (Blood fluke).
• Class Cestoda: This class includes organisms that are primarily endoparasitic. They are commonly called tapeworms.

Common examples of Platyhelminthes

• Tapeworm (Taenia solium)
• Liver fluke (Fasciola hepatica)
• Planaria

Fig: Examples of Platyhelminthes

Phylum Aschelminthes

The name Aschelminthes is derived from the Greek words ‘nema’, which means thread, and ‘helmin’, which means worm. Therefore, the Aschelminthes members are also known as nematodes.

Characteristic features of Aschelminthes

Some of the characteristic features of Aschelminthes are listed below:

• They are also known as roundworms because they have a cylindrical body.
• They live in terrestrial as well as aquatic habitats.
• They have bilateral symmetry which means their body is divided into two equal and identical halves.
• The members of Platyhelminthes have an organ system level of organisation.
• They are triploblastic and have three germ layers.
• A false body cavity is present and therefore they are pseudocoelomates.
• Their digestive system is complete.
• Nervous system consists of a nerve ring around the pharynx with a dorsal and ventral nerve cords.
• They are unisexual and shows sexual dimorphism.
• The males are smaller than the females and possess penial spicules which help in copulation.
• Lifecycles possess larval stages. For example filarial worms present in Ancylostoma and rhabditiform larva present in Ascaris.

Classification of Aschelminthes

Phylum Aschelminthes is classified into two classes:

• Class Aphasmidia: Phasmids and chemoreceptors are absent in this class. In this group of nematodes, there are numerous varieties of amphids or taste receptors. Examples include Trichinella.
• Class Phasmidia: The body's rear end possesses phasmids. Amphids are situated close to the anterior end and contain pore-like structures. Amphids are glands, and their purpose is to serve the gustatory apparatus. This class of organisms has well-developed excretory organs. Examples include Ascaris and Ancylostoma.

Common examples of Aschelminthes

• Ascaris (Roundworm)
• Wuchereria (Filarial worm)
• Ancylostoma (Hookworm)
• Enterobius (Pinworm)

Fig: Examples of Aschelminthes

Phylum Annelida

The Latin words ‘annulus’ which means little ring, and ‘lidos’ which means form, are the roots of the word Annelida. They are called segmented worms.

Characteristic features of Annelida

Some of the characteristics of Annelida are listed below:

• They are either free-living or parasitic organisms and commonly found in terrestrial as well as aquatic environments.
• Their body is well-differentiated into head and tail.
• They are bilaterally symmetrical which means that their body can be divided into two equal halves.
• They possess organ system level of organisation.
• They are triploblastic and therefore, three germ layers are present.
• A true coelom is present between the gut wall and body wall and therefore, they are known as coelomates.
• Their body is externally and internally divided into segments with a serial repetition of organs in it. These body segments are known as metamers.
• They possess setae or parapodia for locomotion.
• Digestive tract is complete.
• They possess a closed circulatory system.
• They possess nephridia as excretory organs.
• Nervous system consists of a nerve ring around the pharynx with ganglia.
• They possess a larval stage called trochophore larva in the life cycle.

Classification of Annelida

On the presence or absence of setae, they are classified into four classes as follows:

• Class Polychaeta: In this class, numerous setae are present in parapodia. Examples include Nereis.
• Class Oligochaeta: They have setae for locomotion but these structures are present in few numbers. Examples include earthworms.
• Class Hirudinea: The members of this class do not have parapodia and setae for locomotion. Examples include Hirudinaria.
• Class Archiannelida: Parapodia and setae, both structures are absent in this class. Examples include Polygordius.

Common examples of Annelida

• Earthworm (Pheretima)
• Nereis (sandworm)
• Blood sucking leech (Hirudinaria)

Fig: Examples of Annelida

Phylum Arthropoda

The greek word ‘Arthropoda’ has been derived from two words ‘arthron’ meaning joint and ‘podos’ meaning foot.

Characteristic features of Arthropoda

Some of the characteristic features of Arthropoda are listed below:

• Two-thirds of all species identified to date on Earth are members of the Arthropoda phylum, which makes them the largest phylum of the Animalia Kingdom.
• They have characteristic jointed appendages.
• They are well-adapted to terrestrial as well as aquatic environments.
• They have a hard chitinous exoskeleton.
• The members of phylum Arthropoda are bilaterally symmetrical.
• They possess organ system level of body organisation.
• They are triploblastic and have three germ layers.
• A true body cavity is present which is lined by the mesoderm and therefore, they are known as coelomates.
• Their body is well-differentiated into the head, thorax, and abdomen.
• The alimentary canal is complete and divided into foregut, midgut and hindgut.
• Respiratory organs are gills in aquatic forms and tracheae or booklings in terrestrial forms.
• Circulatory system is open type.
• Excretory organs are green glands or malpighian tubules.
• Nervous system consists of a nerve ring and double ganglionated ventral nerve cord.
• They possess compound eyes.
• They are unisexual.

Classification of Arthropoda

Arthropods are divided into seven separate classes based on the body divisions and presence or absence of particular appendages.

• Class Crustacea: In this class, their body is divided into cephalothorax, and abdomen. An exoskeleton known as the Carapace (Chitin and CaCO₃) covers the cephalothorax. There are two antennae pairs as well as a pair of complex eyes present. Examples include prawns.
• Class Chilopoda: The body of class Chilopoda is differentiated into head and trunk. The first set of legs has been modified to create poison claws. Examples include Scolopendra.
• Class Diplopoda: The body of these insects is differentiated into head, thorax, and abdomen. There are two pairs of legs in every abdominal segment. Examples include millipedes.
• Class Insecta: The body of class Insecta is divided into the head, thorax, and abdomen. On the thorax region, three pairs of legs and two pairs of wings are attached. The abdomen is devoid of appendages. Examples include housefly.
• Class Arachnida: The body is differentiated into cephalothorax (head and thorax) and abdomen. They have one pair of chelicerae, one pair of pedipalpi, and four pairs of legs. Their antennas are absent. Examples include spiders.
• Class Onychophora: It is a connecting link between Annelida and Arthropoda. They have a worm-like segmented body. Examples include Peripatus.
• Class Merostomata: It is a living fossil and has book gills that act as their excretory structures. Examples include horseshoe crab.

Common examples of Arthropoda

• Anopheles, Culex and Aedes (Mosquito)
• Silkworm (Bombyx mori)
• Lac insect (Laccifer)
• Honeybee (Apis)
• Locust (Locusta)

Fig: Examples of Arthropoda

Phylum Mollusca

The word Mollusca comes from the Latin words ‘mollis’ which means ‘soft.’ The creatures with soft bodies belong to this phylum.

Characteristic features of Mollusca

Some of the features of Mollusca are listed below:

• It is known as the second largest phylum of the Kingdom Animalia.
• They are found in terrestrial as well as aquatic environments.
• The members of this phylum have a hard outer calcareous shell.
• They are bilaterally symmetrical.
• They possess organ system level of organisation.
• They are triploblastic and have three germ layers.
• A true coelom is present and therefore, known as coelomates.
• The body is divided into a head, visceral hump and foot.
• They possess a rasping organ called radula.
• They possess feathery gills or ctenidia for respiration.
• They possess an open circulatory system.
• Excretory system consists of Bojanus organs.
• Sexes are separate.
• Lifecycles possess larval stages like glochidium, veliger or trochophore larvae.

Classification of Mollusca

The phylum Mollusca is classified into six classes:

• Class Monoplacophora: Their shell is either dome-shaped or spoon-shaped. They possess features of Annelida and Mollusca. Examples include Neopilina.
• Class Amphineura: In this class, the shell is either present or absent. They have a nerve ring around the mouth. Examples include Chiton.
• Class Scaphopoda: The shell of the members is tubular in structure and is open at both ends. Examples include Dentalium.
• Class Gastropoda: It is the largest class of Mollusca. A single shell is present that is spirally coiled. They are asymmetrical. Examples include Pila.
• Class Pelecypoda or Bivalvia: The members of this class have two shells. Examples include Unio.
• Class Cephalopoda: The shell in these members is either external or internal in position. Examples include Sepia.

Common examples of Mollusca

• Chiton (Chaetopleura)
• Cuttlefish (Sepia)
• Pearl oyster (Pinctada)
• Devil fish (Octopus)
• Tusk shell (Dentalium)
• Sea hare (Aplysia)
• Apple snail (Pila)
• Squid (Loligo)

Fig: Examples of Mollusca

Phylum Echinodermata

The word Echinodermata is derived from the Greek words ‘echinos’ which means spiny, and ‘derma’ which means skin or covering. Like porcupines, echinoderms have spiny skin.

Characteristic features of Echinodermata

Some of the features of Echinoderms are listed below:

• It is the only phylum with a large number of stenohaline members, and they are well adapted to the marine conditions.
• They have calcareous ossicles as their endoskeleton.
• Adult echinoderms show radial symmetry and larvae of echinoderms show bilateral symmetry.
• They have an organ system level of organisation.
• They are triploblastic animals and have three germ layers.
• A true body cavity is present and therefore, they are known as coelomates.
• They possess ambulacral systems.
• Circulatory system is open type.
• Nervous system consists of a circum-oral ring and radial nerves.
• Sexes are separate.
• They show autotomy and regeneration.

Classification of Echinodermata

The animals in this phylum Echinodermata are divided into five classes. They are as follows:

• Class Asteroidea: The body is shaped like a star and is flat. With a central disc, they have five thick, non-sharp limbs. Examples include starfish.
• Class Ophiuroidea: Here, the body shape is flat and resembles a star. They have five slender, long, jointed, and brittle arms. Examples include Ophiothrix.
• Class Echinoidea: The body is globular and disc-like in shape. They have no arms. They move by using their long, moveable spines, which are employed for propulsion. Examples include Echinus.
• Class Holothuroidea: The body is cylindrical and long. These animals don't have arms. Examples include Cucumaria.
• Class Crinoidea: The body form is attached to the substrate in a plant-like manner. They possess a number of arms that emerge from a calcareous plate. Each arm splits into two parts and has lateral pinnules. Examples include Antedon.

Common examples of Echinodermata

• Asterias (Starfish)
• Ophiothrix (Brittle star)
• Echinus (Sea urchin)
• Cucumaria (Sea Cucumber)
• Antedon (Sea lily)

Fig: Examples of Echinodermata

Phylum Hemichordata

Hemichordata was formerly included under the phylum Chordata but is now considered to be a non-chordate. The terms ‘hemi’ which means ‘half’ and ‘chordata’ which means ‘notochord’ are combined to form the word ‘hemichordata.’ They are therefore called half chordates and they resemble little worm-like creatures.

Characteristic features of Hemichordata

Some of the features of Hemichordata are listed below:

• The body is undifferentiated, worm-like, soft, and brittle. It is divided into the collar, trunk, and proboscis.
• Hemichordates have a primitive structure called a stomochord in their collar area that is extremely similar to a notochord. It is a hollow, flexible tube.
• They are capable of living in marine conditions and therefore, they respire through gills.
• The members of phylum Hemichordata are bilaterally symmetrical.
• They have an organ system level of body organisation, where there is a clear division of labor among the many organ systems.
• They are triploblastic which means that three germ layers are present.
• The members of this phylum have separate sexes and external fertilisation is commonly seen.
• Circulatory system consists of a dorsal heart and two longitudinal vessels such as dorsal and ventral vessels.
• Development is indirect and includes a free swimming tornaria larva.

Classification of Hemichordata

The members of the phylum Hemichordata are classified into three classes:

• Class Enteropneusta: They are also known as acorn worms. The circulatory system of acorn worms includes a heart that resembles a kidney. They breathe using gill-like structures that resemble the gills of early fish. They are sometimes referred to as the connection between invertebrates and vertebrates.
• Class Pterobranchia: They use cilia attached to tentacles for feeding. Plankton from the water is filtered through it.
• Planctosphaeroidea: Only the larvae of this species are known. They show unrestricted swimming. The tornaria larva shows similarity with them. They have a ciliated band to collect food particles.

Common examples of Hemichordata

• Balanoglossus gigas
• Saccoglossus

Fig: Examples of Hemichordata

Phylum Chordata

The phylum Chordata possess animals that have notochords.

Characteristic features of Chordata

Some of the defining features of phylum Chordata are listed below:

• A dorsal notochord is present which is mesodermal in origin.
• A dorsal hollow nerve cord is present in these animals.
• At some point in their development, all chordates possess paired pharyngeal gill slits.
• Post-anal tail is present which is described as the body’s elongation on the post-anal region.
• Chordates have a heart with chambers. It is on the ventral side of the body.
• Chordates can survive and thrive in a variety of terrestrial, aquatic (marine or freshwater), or combined habitats.
• They are bilaterally symmetrical.
• In the organ system level of body organisation of chordates, there is a clear division of labor between the various organ systems.
• They are triploblastic and have ectoderm, mesoderm, and endoderm.
• In chordates, a body cavity exists between the body wall and gut wall, which is bordered by mesoderm or coelomic epithelium. Chordates are coelomates because of the presence of coelom.

Classification of Chordates

Phylum Chordata is further classified into three subphyla as follows:

• Urochordata or Tunicata
• Cephalochordata
• Vertebrata

Urochordata or Tunicata

Subphylum Urochordata is distinguished by the exclusive presence of notochord in the larval tail. In adulthood, it fades away. The nerve cord, which is present in larvae, is replaced by a dorsal ganglion in adults. Examples of urochordates include Doliolum, Ascidia, Salpa, Herdmania, etc.

Fig: Examples of Urochordates

Cephalochordata

A notochord that runs from the head to the tail region is a distinguishing feature of subphylum Cephalochordata. The subphylum is known as Cephalochordata because the notochord extends to the body's anterior tip. In cephalochordates, the notochord remains throughout life. Examples include Branchiostoma (Sea lancelet or Amphioxus).

Fig: Example of Cephalochordata - Branchiostoma

Vertebrata or Craniata

The highest order of chordates are vertebrates. It is distinguished by the single notochord present during the embryonic stage of life, which eventually develops into a cartilaginous or bony vertebral column in the adult form. An anterior notochord segment is replaced with a skull or cranium to protect the brain. The vertebrates are also referred to as Craniata for this reason.

Fig: Development of notochord into a vertebral column in vertebrates

Common examples of Chordates

• Reptiles
• Fishes
• Amphibians
• Mammals
• Birds

Fig: Common examples of Chordata

Practice Problems

1. Which is the second largest phylum in the Animal Kingdom?

1. Arthropoda
2. Annelida
3. Chordata
4. Mollusca

Solution: The largest phylum in the Animal Kingdom is called Arthropoda. Approximately 84% of the animal species on Earth are represented by it. Mollusca is the second largest phylum of the Kingdom Animalia. There are numerous species in this phylum that are freshwater, marine, and terrestrial. About 100,000 live species are present there. Hence, the correct option is d.

2. In the animal kingdom, how many phyla have radial symmetry?

1. 1
2. 2
3. 3
4. 4

Solution: Any plane passing through the centre of the body which splits the organism into two identical halves is called radial symmetry. The organisms that possess radial symmetry are called radiata. In the animal kingdom, radial symmetry is exhibited by three phyla as follows:

• Cnidaria
• Ctenophora
• Echinodermata

Hence, the correct option is c.

3. From the given options, which is the most distinctive feature that distinguishes non-chordates from chordates?

1. Setae
2. Notochord
3. Ventral tubular nerve cord
4. Gills and lungs

Solution: Those animals which do not possess a notochord at least some stage in their life cycle are referred to as non-chordates. Examples include starfish, cockroaches, earthworms, etc. Those animals which possess a notochord at least some stage in their life cycle are known as chordates. It comprises fishes, amphibians, reptiles, birds, mammals, etc. The vertebral column replaces the notochord in higher chordates or vertebrates. Hence, the correct option is b.

4. Which taxonomic unit is exclusively used for the classification of the Animal Kingdom?

Answer: The exclusively used taxonomic unit for the classification of the Animal Kingdom is phylum. Phylum is described as a group of related animals.

FAQs

1. Which animal is considered immortal?

Answer: There is only one species that has been called biologically immortal and this is the jellyfish, Turritopsis dohrnii. These tiny, translucent creatures, which are found in waters all over the world, have the ability to go back in time and resume an earlier phase of their life cycles.

2. What is the reason behind the immense success of arthropods?

Answer: The arthropods have multipurpose appendages that allow more mobility on land. Arthropods may be found in almost every environment and niche on the Earth because of the evolution of a wide variety of appendages, including antennae, claws, wings, and mouthparts. They have a hard exoskeleton of chitin that protects the body.

3. Which animals have the most powerful bite in the world?

Answer: Hippopotamus, saltwater crocodiles, great white sharks, etc., have the most powerful bites in the world.

4. How much time does a sloth need to digest a food?

Answer: A sloth needs a minimum of two weeks to digest food. This is because it has the lowest metabolic rate which means that it takes a long time to digest its meal.

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