Basis of Animal Classification
* Animals are classified on the basis fundamental features such as level of organisation, symmetry, cell organisation, nature of coelom, segmentation and notochord.
* Level of organisation is the first basic fundamental feature by which animals are classified. There are four main types of cell patterns: cellular, tissue, organ and organ system.
* Some animals are symmetrical, some have radial symmetry and many exhibit bilateral symmetry.
* In animals, there are two types of cell organisation-diploblastic and triploblastic.
* Acoording to the nature of coelom, animals are classified as coelomates, acoelomates and pseudocoelomates.
* Animals are also classified on the basis of segmentation.
* The most important feature for animal classification is the presence or absence of a notochord.
Non-chordates I
* Animals belonging from Phylum Porifera to Echinodermata are categorised as nonchordates.
* Non-chordates are animals without a notochord.
* Class Porifera includes multi-cellular animals with cellular level of organisation and characteristic flagellated choanocytes.
* Coelenterates are mostly aquatic, sessile or free-floating with cnidoblasts in their tentacles.
* Ctenophores are marine animals with comb plates.
* A characteristic feature of ctenophores is ‘bioluminescence’.
* Platyhelminthes have a flat body and exhibit bilateral symmetry.
* The parasitic members of Platyhelminthes have suckers and hooks.
* Aschelminthes include parasitic as well as non-parasitic roundworms.
Non-chordates II
* Annelids are bilaterally symmetrical, metamerically segmented animals with the true coelom.
* Arthropods are characterised by the presence of jointed appendages and chitinous exoskeleton.
* Molluscs have a soft body surrounded by an external calcareous shell.
* The body of a mollusc consists of a distinct head, a muscular foot and a visceral hump.
* Echinoderms have a spiny skin with a distinctive water vascular system.
Hemichordata and Chordata
* Hemichordates are a small group of worm-like marine animals with a cylindrical body divided into proboscis, collar and trunk.
* Phylum Chordata includes animals which possess a notochord either throughout or during early embryonic life.
* Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata.
* Subphyla Urochordata and Cephalochordata are also referred to as protochordates.
* Urochordates have a notochord only in the larval tail, while cephalochordates have a persistent notochord, which extends from head to tail region.
* Vertebrates possess a notochord during the embryonic period, which is later replace by a cartilaginous or bony vertebral column.
Vertebrates
* All vertebrates are chordates but all chordates are not vertebrates.
* Cyclostomates are the most primitive chordates, without jaws and are ectoparasites on fish.
* Chondrichthyes are marine fish with a cartilaginous endoskeleton and a persistent notochord.
* Osteichthyes are marine and freshwater fish with a bony endoskeleton and streamlined body.
* Amphibians have adapted to both land and water.
* Reptiles are characterised by the presence of dry and cornified skin.
* Birds are warm-blooded animals and have feathers on their bodies.
* Mammals have mammary glands, hair on their skin and are warm-blooded animals.
