Kingdom Animalia includes approximately 1.2 million species of multi-cellular animals.
GENERAL CHARACTERISTICS OF ANIMALIA
- All organisms included in Kingdom Animalia are Eukaryotic and possess a well-defined nucleus and membrane-bound organelles
- They have a heterotrophic mode of nutrition and rely on exogenous sources for food.
- They possess a multicellular organization.
- The cell wall is absent in the cells of these organisms. The cell membrane forms the outermost boundary of cells in Animalia.
- Reproduction is generally sexual. Gametes represent the only haploid stage in these animals.
CLASSIFICATION OF ANIMALIA
BASIS OF CLASSIFICATION
Based on specific features, the animal kingdom has been classified into 11 different phyla.
There are few common features, which form the basis for animal classification. These features are explained below.
LEVEL OF ORGANIZATION
- Though all members of Kingdom Animalia are multicellular, the pattern of cellular organization varies from species to species.
- Different levels of organization are as follows;
- In this level of organization, the organism’s body consists of a mass of protoplasm confined within the cell membrane.
- All the activities of life are performed within the protoplasm by various cell organelles.
- Such organization is present in Protozoa
- The body of an organism consists of multiple cells, which are either arranged into an aggregate or a colony.
- Cellular level of organization further consists of two types
- Cellular aggregates i.e. sponges (Porifera)
- Cellular colonies i.e. Protists and some algae.
- Similar cells are grouped to form tissues which are specialized to carry out specific functions.
- This level of organization can be observed in Coelenterates.
- Tissues join together and function as a single unit forming an organ
- organ level of organization is found in Platyhelminthes.
- Organ-system level of organization occurs when various organs are grouped into a functional system.
- Each system is specified to perform a certain physiological function.
- It is found in animals like Arthropods, Annelids, Mollusks, Echinoderms, and Chordates.
- The complexity of organ systems varies among animals, on the basis of which they are grouped into different phyla.
- Organs such as stomach, intestines etc. together constitute the digestive system.
- Such level of organization can be seen in Nemathelminthes to chordates
- symmetry refers to a correspondence of body parts in all major respects like size, shape, and relative position, on opposite sides of a dividing line or distributed around a central point or axis
- two types of symmetry are found in animals;
- Radial symmetry
- Bilateral symmetry
- When any plane passing through a central point divides the animal into two identical halves, this is known as radial symmetry.
- Coelenterates, Ctenophores, and Echinoderms display radial symmetry
- Body is divided into two identical left and right halves only when a plane passes through the median longitudinal axis
- FOR EXAMPLE
Annelids, Arthropods and Mollusks display bilateral symmetry.
- Some animals are asymmetrical, which means that any plane passing through a central point does not divide the animal into equal halves.
- Germ layers are primary cell layers formed during the earliest stages of embryonic life
- Germ layers are three in number i.e. ectoderm, endoderm and mesoderm
- Based on the number of germ layer, animals are grouped into two categories namely;
- Embryo of diploblastic animals consists of two germ layers i.e. an outer ectoderm and an inner endoderm
- Hydra, Jellyfish etc.
- Embryo of triploblastic animals consists of three germ layers i.e. an outer ectoderm, a middle mesoderm and inner endoderm.
- Coelom is the principal body cavity of an animal, lying between the outer body wall and inner digestive tube.
- on the basis of types of coelom, the animals are grouped into four categories namely;
- Eu-coelomates (true coelom)
- In case of acoelomates, there is no body cavity.
- The region between the outer body wall (epidermis- made from ectoderm) and the inner digestive tract (made from endoderm) is completely filled with mesoderm in the form of a spongy mass of cells known as the parenchyma.
- Pseudocoelomates are characterized by the presence of a false coelom or perivisceral cavity.
- The pseudocoelom is not lined by mesoderm and is directly connected to the archenteron.
- Eu-coelomates consists of a true body cavity lined with mesoderm.
- Segmentation refers to the serial repetition of similar parts along the length of the animal.
- According to segmentation, animals may be grouped into two categories;
- In this type of segmentation, the animal’s body is divided into a number of pseudo-segments, which are independent of each other.
- Metameric type of segmentation refers to the linear repetition of body parts
- Notochord is a rod-like structure of mesodermal origin, present on the dorsal side of the animal body.
- Based on the presence and absence of notochord, animals are grouped into two categories namely;
- Chordates (notochord is present)
- Non-Chordates (notochord is absent)
- Arihant’s handbook of biology. Animal Kingdom. Page no: 43-73.
- NCERT biology; textbook for class 11. Animal Kingdom. Page no: 46-62.