Animal Kingdom Notes Class 11 Biology Chapter 4

→ The animal kingdom is the most diverse group of organisms, which include about 1.2 million living species. Animals are heterotrophic organisms, which ingest food. Most animals move about in search of food. Some are sedentary or remain fixed at a place. In the food enters in their body by the water current. These two methods of feeding, shape the anatomy and behavior of animals to a great extent.

→ The major aim of classification is to express the relationship existing between different organisms. Based on certain characteristics, organisms are grouped together and separated from unrelated ones.

→ Over a million species of animals have already been described and many more are yet to be discovered. Basic fundamental features such as levels of the organization, symmetry, cell organization, body cavity and coelom, segmentation, etc., have enabled us to broadly classify the animal kingdom to a certain extent. Besides the fundamental features, there are many other distinctive characters, which are specific for each phylum or Class.

→ Porifera includes multicellular animals which exhibit a cellular level of organization and have characteristic flagellated collar cells (Choano- cytes). The cnidarians have tentacles and bear cnidoblasts. They are mostly aquatic, being sessile or free-floating. The ctenophores are marine animals with comb plates.

The platyhelminths have a flat body and exhibit bilateral symmetry. The parasitic forms show distinct suckers and hooks. Aschelminthes include parasitic as well as non-parasitic roundworms.

→ The annelids are metamerically segmented animals with a true coelom. The mollusks have a soft body surrounded by an external calcareous shell. The arthropods are the most successful group of animals characterized by the presence of jointed appendages.

The body is covered with an external skeleton made of chitin. The echinoderms possess spiny skin. Their most distinctive feature is the presence of a water vascular system. The hemichordates are a small group of worm-like marine animals. They have a cylindrical body with proboscis, collar, and trunk. This phylum was earlier considered as a subphylum under the phylum Chordata.

→ The phylum, Chordata, includes animals, which possess a notochord either throughout or during early embryonic life. Other common features observed in the chordates are the dorsal, hollow nerve cord and the paired pharyngeal gill-slits. Some of the vertebrates do not possess jaws (Agnatha) whereas most of them possess jaws (Gnathostomata). Agnatha is represented by the class Cyclostomata. They are the most primitive chordates and are ectoparasites on fishes. Gnathostomata has two superclasses Pisces and Tetrapoda.

→ Classes Chondrichthyes and Osteichthyes bear fins for locomotion and are grouped under Pisces. The Chondrichthyes are fishing with cartilaginous endoskeleton and are mostly marine. Whereas, classes Amphibia, Reptilia, Aves, and Mammalia, move with the help of two pairs of limbs and are thus grouped under Tetrapoda.

→ The amphibians are adapted to live both on land and water. Reptiles are characterized by the presence of dry and cornified skin. Limbs are absent in snakes. Fishes, amphibians, and reptiles are poikilothermous (cold¬blooded).

→ Aves, commonly known as birds, are warm-blooded animals with forelimbs modified into wings for flying and hind limbs adapted for walking, swimming, perching, or clasping.

→ The unique features of mammals are the presence of mammary glands and hairs on the skin. They exhibit viviparity. Mammals are the most intelligent among all the animals.

→ Radial Symmetry: When any plane passing through the central axis of the body divides the organism into halves that are approxi¬mately mirror images, it is called radial symmetry.

→ Bilateral symmetry: Animals like annelids, arthropods, etc., where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry.

→ Diploblastic: Animals, in which the cells are arranged into two embryonic layers, external ectoderm and internal endoderm, are called diploblastic

→ Triploblastic: Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm are called triploblastic animals.

→ Coelom: The body cavity, which is lined by mesoderm is called coelom.

→ Eucoelomates: Animals possessing coelom are called coelomates.

→ Pseudocoelomates: Such a body cavity is called pseudocolor and the animals possessing them are called pseudocoelomates.

→ Metamerism: In some animals, the body has many segments, which show serial repetition of the phenomenon is known as metamerism.

→ Polyp and medusa: Cnidarians exhibit two basic body forms called Polyp and Medusa.

→ Flatworms: Platyhelminthes are Dorso ventrally flattened and, hence, commonly known as flatworms.

→ Brain ganglion: Hooks and suckers are present in the parasitic forms. They possess a concentration of nervous tissue in the head called the brain ganglion.

→ Flame cells: Specialized cells called flame cells help in osmo¬regulation and excretion.

→ Radula: Mollusca mouth contains file-like rasping organs for feeding, called a radula.

→ Metamorphosis: The process of transformation of a larva into an adult is called metamorphosis.

→ Cloaca: Alimentary canal, urinary, and reproductive tracts open into a common chamber called the cloaca, which opens to the exterior through an aperture called the cloacal aperture.