Class 11 Biology Chapter 4 emphasizes the importance of the Animal Kingdom. Kingdom Animalia is the most prominent and significant classification made in the Five-Kingdom Scheme of Classification by ecologist R.H. Whittaker.

Animals are metazoans. There are multicellular, eukaryotic, and heterotrophic animals. All these animals are classified based on their standard fundamental features:

• • The cellular arrangement.

  • Body symmetry.

  • Presence/absence of the coelom.

  • Distinct features of the digestive system, reproductive, and circulatory system.

Animal bodies have multiple cells with mitochondria, and they are dependent on other organisms for their food. Animal cells are organized into tissues that perform specific functions. Animals have several systems like the organ systems, muscular system, circulatory system, respiratory system, skeletal system, digestive system, immune system, endocrine system, excretory system, and reproductive system, which, when put together, make them functional. 

Class 11 Biology Chapter 4 NCERT Solutions puts together a comprehensive learning aid for students to know more about the Animal Kingdom.

The basis of classification in animals is done using these few primary features:

1. Symmetry

There are two types of bases of symmetry in animals:

Bilateral symmetry: Here, only a single plane divides the body into two halves.

Example: Arthropods, Annelids, Molluscs, etc.

Radial symmetry: In this type of symmetry, any plane going along the central axis divides the body into two halves.

Example: Coelenterates, Echinoderms, Ctenophores

2. Organizational levels

Animals are multicellular beings. The level of organization of cells differs from animal to animal. For example, poriferans have a loose structure of cells. They show the cellular level of organization. Upon moving forward, coelenterates have the tissue level of organization. An organ level of organization is present in phylum Platyhelminthes and Aschelminthes. A specialized organ system accentuates physiological activities among non-chordates such as Annelids, Molluscs, Arthropods, Chordates, And Echinoderms. So, the various levels of the organization are:

1. Cellular level

2. Tissue level

3. Organ level

4. Organ system (specialized organ) level

3. Diploblastic and triploblastic organization

Animals are basically classified as diploblastic and triploblastic animals based on the number of embryonic layers.

Diploblastic animals: In these animals, the cells form two embryonic layers, i.e., an inner layer, endoderm, and an outer layer, ectoderm.

Examples: Phylum Coelenterata

Triploblastic animals: The cells are arranged in three embryonic layers: ectoderm, endoderm, and mesoderm (a middle layer).

Examples: Phylum Chordata and Phylum Platyhelminthes

4. Coelom

A coelom is a hollow, fluid-filled tube that is found in many living creatures and functions as a cover for their vital organs.

Coelomate: In coelomates, the coelom is present. The coelom is a body cavity between the body and gut walls and is lined by a mesoderm. Based on the presence and absence of coeloms, animals are classified into:

Example: Phylum Annelida, Phylum Mollusca

Acoelomate: In acoelomates, the coelom is absent.

Example: Phylum Platyhelminthes

Pseudocoelomate: Among pseudocoelomates, the mesoderm appears as a pouch between the endoderm and ectoderm. The cavity formed is not lined by mesoderm.

Example: Phylum Aschelminthes

Here are the classifications of animals in Kingdom Animalia and their standard features.

Porifera (Sponges)

Features:

• • Marine, asymmetrical, have a cellular level of organization. 

  • Asexual reproduction by fragmentation.

  • Sexual reproduction involves forming gametes.

  • Food intake, respiration, and excretion are processed via a water transport system.

  • Internal fertilization and the development of the zygote go through a larval stage.

  • Examples: Sycon, Euspongia (bath sponge), Spongilla (freshwater sponge).

Coelenterata (cnidaria)

Cnidarian, also known as coelenterate, is any member of the phylum Cnidaria (Coelenterata), containing over 9,000 living species. Jellyfish and Hydra are two common examples of this species.

Features:

• • Aquatic, radially symmetrical, have a tissue level of organization

  • Acoelomate, diploblastic.

  • Sensory tentacles.

  • The tentacles have cnidoblasts containing nematocysts.

  • Examples: Adamsia (sea anemone), Meandrina (brain coral), Gorgonia (sea-fan)

Ctenophora 

Ctenophora is a group of marine invertebrates that comprises comb jellies that survive in saltwater all around the world. They are famous for the groups of cilia they employ for swimming, and they are also the largest animals to swim using cilia.

Features:

• • Marine, radially symmetrical, tissue level of organization.

  • Acoelomate and diploblastic.

  • Sexual reproduction and fertilization are external.

  • Are bioluminescent.

  • Hermaphrodite.

  • Examples:  Ctenoplana and Pleurobrachia

Platyhelminthes (flatworms)

The following characteristics distinguish Platyhelminthes: They are acoelomate, triploblastic, and bilaterally symmetrical. They can be either free-living or parasitic. Its body has a delicate coating that may or may not have cilia.

Features:

• • The parasites have hooks and suckers.

  • Internal fertilization and indirect development occur through multiple larval stages.

  • Examples: Taenia (tapeworm), Fasciola (Liver fluke).

Aschelminthes

The Aschelminthes, which are closely connected to the Platyhelminthes, is an ancient species of pseudocoelomate and other similar animals that are no longer closely related and have been promoted to their classes.

Features:

• • Aquatic or terrestrial.

  • Free-living or parasitic.

  • Triploblastic.

  • Pseudocoelomate.

  • Organ system level of organization.

  • Bilaterally symmetrical.

  • Internal fertilization.

  • Examples: Wuchereria or Filarial worm, Ascaris or roundworm, Ancylostoma.

Annelida

The annelids, commonly known as ringed worms or segmented worms, are a diverse species, including ragworms, earthworms, and leeches.

Features:

• • Bilaterally symmetrical and coelomate.

  • Triploblastic and organ system organization.

  • Sexual reproduction. Earthworms and leeches are monoecious in nature, while Nereis is dioecious in nature.

  • Examples: Pheretima, Hirudinaria (leech), Nereis, etc.

Arthropoda

Arthropods are invertebrates with an exoskeleton, segmented bodies, and paired jointed legs. Arthropods are classified as members of the phylum Arthropoda. They are recognized by their jointed limbs and chitin cuticle, which is frequently mineralized with calcium carbonate.

Features:

• • It is the largest phylum consisting of two-thirds of animals. The phylum includes insects.

  • Organ system level of organization, Bilaterally symmetrical, coelomate, and triploblastic.

  • Examples: Bombyx (silkworm), mosquitoes like Anopheles, Aedes, Culex, Apis (honey bee), etc.

Mollusca

After the Arthropoda, Mollusca is the second-largest phylum of invertebrates.

Features:

• • Triploblastic and Bilaterally symmetrical.

  • Coelomate with organ system organization.

  • Their bodies are covered with a calcareous shell.

  • Distinct head, muscular foot, and the visceral hump are present.

  • Examples: Pila (apple snail), Octopus, Pinctada (pearl oyster).

Echinodermata

An echinoderm refers to a member of the phylum Echinodermata. Echinoderm adults are distinguished by their radial symmetry, including starfish, sea urchins, and sea cucumbers.

Features:

• • Symmetrical.

  • Triploblastic and coelomate.

  • Dioecious, external fertilization with indirect development.

  • Examples: Asterias (starfish), Echinus, and Ophiura (brittle star).

Hemichordata

Hemichordata is a phylum of marine deuterostome organisms that is commonly regarded as the echinoderms' sister group. They appeared in the Lower or Middle Cambrian period and are divided into two groups: Enteropneusta and Pterobranchia.

Features:

• • Bilaterally symmetrical.

  • It consists of a stomochord, which is a structure similar to the notochord.

  • Triploblastic and coelomate in nature.

  • Examples: Balanoglossus, Saccoglossus.

Chordata

A chordate is a member of the Chordata phylum. At some point during their larval or maturity stages, all chordates have five synapomorphies or main traits that separate them from all other taxa.

Features:

• • Bilaterally symmetrical.

  • Characteristic features include a dorsal hollow nerve cord, notochord, paired gill slits, endostyle or thyroid, posterior tail.

  • Triploblastic and coelomate. 

  • Example: Pandas, crows, fishes.

1. Why are arthropods the biggest phylum among the animal kingdom?

Arthropods are the largest species, and they consist of over 80% of the animal kingdom. They have certain adaptive qualities that they have evolved over the years to survive in changing climates. They cover two-thirds of the entire animal kingdom. This large phylum, especially terrestrial arthropods, can adapt to problems regarding support, stability, respiration, and desiccation. 

Arthropods like cockroaches can survive under conditions of nuclear radiation and toxic environmental conditions like poisoned soil, etc. All these factors make arthropods the largest phylum among animals.

Example: Limulus, Mosquitos, etc.

2. List certain animals that are found parasitic on human beings from Chapter 4 of NCERT Solutions for Class 11 Biology.

Some of the parasitic animals found in human beings, as given in Animal Kingdom Class 11 Ncert Solutions, are:

• • Ascaris (Roundworm)

  • Enterobius (Pinworm)

  • Ancylostoma (Hookworm)

  • Taenia (Tapeworm)

  • Wuchereria (Filarial worm)

 3. What questions can I expect from Lesson 4 of NCERT Solutions for Class 11 Biology in the exam?

Class 11 Biology Chapter 4 Solutions contain everything you can expect in your exams, including objective and short answers. If you want to know the questions expected to come in the exam, check out the previous year's question papers and thoroughly work on them for the best results. This chapter briefly explains the differences in the structure and form of animals.