CHAPTER 4 :ANIMAL KINGDOM ● Animal species are highly diverse; over 1 million species described. ● Classification helps in: ○ Studying animals systematically. ○ Assigning positions to new species. BASIS OF CLASSIFICATION Animals are classified based on fundamental features such as: ● Levels of organisation ● Body symmetry ● Germ layers ● Coelom ● Segmentation ● Presence/absence of notochord ● Type of digestive/circulatory system 1. Levels of Organisation All animals are multicellular, but vary in complexity: 1. Cellular Level: ○ Found in sponges (Porifera). ○ Cells loosely arranged with little coordination. ○ No tissues or organs. 2. Tissue Level: ○ Found in Coelenterates (Cnidarians). ○ Similar cells form tissues performing specific functions. 3. Organ Level: ○ Seen in Platyhelminthes and higher phyla. ○ Tissues group into organs for specific functions. 4. Organ System Level: ○ Found in Annelids, Arthropods, Molluscs, Echinoderms, Chordates. ○ Organs work together as organ systems. ○ Digestive system: ■ Incomplete: Single opening (mouth = anus) → e.g., Platyhelminthes ■ Complete: Separate mouth and anus ○ Circulatory system: ■ Open: Blood directly bathes organs (e.g., Arthropods) ■ Closed: Blood flows in vessels (e.g., Annelids) 2. Symmetry A. Asymmetrical: ○ No plane divides body into equal halves. ○ Example: Sponges(porifera) B. Radial Symmetry: ○ Body divided into identical halves along multiple planes from central axis. ○ Examples: Coelenterates, Ctenophores, Adult Echinoderms C. Bilateral Symmetry: ○ Body divided into two equal halves only in one plane. ○ Examples: Annelids, Arthropods, Vertebrates 3 Germ Layers The layer of gastrula from which body organs are formed. 1. Diploblastic: ○ Body derived from two germ layers: ectoderm and endoderm. ○ Middle layer = mesoglea (non-cellular). ○ Example: Coelenterates 2. Triploblastic: ○ Body derived from three germ layers: ectoderm, mesoderm, endoderm. ○ Examples: Platyhelminthes to Chordates 4 Coelom (Body Cavity) ● Coelom: A fluid-filled body cavity located between the body wall and the gut wall. ● It plays a key role in the classification of animals. Types of Coelom: 1. Coelomates (True Coelom): ○ Coelom is completely lined by mesoderm. ○ Found in: Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates, Chordates 2. Pseudocoelomates: ○ Coelom is not lined by mesoderm. ○ Mesoderm is scattered in the form of pouches between ectoderm and endoderm. ○ Found in: Aschelminthes (e.g., roundworms) 3. Acoelomates: ○ No body cavity between body wall and gut. ○ Found in: Platyhelminthes (flatworms) 5. Segmentation (Metamerism) ● Body is externally and internally divided into repeated segments. ● Some organs are repeated in each segment. ● Example: Earthworm (Annelida) ● This feature is called metamerism. 6 Notochord ● Notochord: A rod-like structure formed from mesoderm, present on dorsal side. ● Appears during embryonic development. ● Animals with notochord: Chordates ● Animals without notochord: Non-chordates (e.g., Porifera to Echinoderms)