Animal Kingdom

The Animal Kingdom comprises multicellular, heterotrophic eukaryotes lacking cell walls, classified using several fundamental criteria: levels of body organization (cellular, tissue, organ, organ-system), body symmetry (asymmetrical, radial, bilateral), number of germ layers (diploblastic — two layers; triploblastic — three layers), presence and type of body cavity (acoelomate — no cavity; pseudocoelomate — false cavity between mesoderm and endoderm; coelomate — true cavity lined by mesoderm), segmentation (metameric or non-segmented), and presence of a notochord.

Porifera (sponges) exhibit cellular-level organization with asymmetrical or radial symmetry. They possess a central spongocoel lined by choanocytes (collar cells) that generate water currents for filter feeding. The skeleton consists of spicules (CaCO₃ or SiO₂) or spongin fibres. Examples: Sycon, Euspongia (bath sponge), Spongilla (freshwater sponge).

Cnidaria (Coelenterata) show tissue-level organization, radial symmetry, and are diploblastic. Their defining feature is cnidocytes containing nematocysts (stinging cells) used for defense and prey capture. They exhibit polymorphism — alternating between polyp (sessile, e.g., Hydra) and medusa (free-swimming, e.g., Aurelia/jellyfish) forms. They have a gastrovascular cavity (incomplete gut). Examples: Physalia (Portuguese man-of-war), Pennatula (sea pen), Meandrina (brain coral).

Ctenophora are also diploblastic with radial symmetry but distinguished by bioluminescence and comb plates (ctenes) used for locomotion. Examples: Pleurobrachia, Ctenoplana.

Platyhelminthes (flatworms) are bilaterally symmetrical, triploblastic, and acoelomate (no body cavity). They are dorso-ventrally flattened with flame cells for excretion. Examples: Planaria (free-living), Taenia solium (tapeworm — parasitic), Fasciola hepatica (liver fluke — parasitic).

Aschelminthes (Nematoda/roundworms) are triploblastic and pseudocoelomate. They have a cylindrical body with a complete digestive system (mouth to anus). Examples: Ascaris lumbricoides (roundworm), Wuchereria bancrofti (causes filariasis/elephantiasis), Ancylostoma (hookworm).

Annelida are triploblastic, coelomate, and exhibit metameric segmentation. They have a closed circulatory system (in some) and nephridia for excretion. Examples: Nereis (polychaete — marine), Pheretima posthuma (earthworm — terrestrial), Hirudinaria (leech — ectoparasite, secretes anticoagulant hirudin).

Arthropoda is the largest phylum in the Animal Kingdom. They are coelomate with jointed appendages, a chitinous exoskeleton (moulted periodically), an open circulatory system (haemocoel), Malpighian tubules for excretion, and compound eyes. Examples: Apis mellifera (honey bee), Bombyx mori (silkworm), Limulus (king crab — a living fossil), Periplaneta americana (cockroach).

Mollusca is the second largest phylum. They have a soft body typically covered by a calcareous shell, a mantle that secretes the shell, and a radula (rasping organ) for feeding. Most have an open circulatory system except Cephalopoda (Octopus, Loligo), which uniquely possess a closed circulatory system. Examples: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Dentalium (tusk shell).

Echinodermata show radial symmetry in adults (bilateral in larvae). They are coelomate with an endoskeleton of calcareous ossicles and a unique water vascular system operating tube feet for locomotion, feeding, and respiration. They possess remarkable regeneration ability. Examples: Asterias (starfish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber).

Hemichordata are bilateral, coelomate, worm-like organisms with a stomochord (a rudimentary structure once thought homologous to the notochord). Example: Balanoglossus.

Chordata is defined by four features present at some stage of life: a notochord, dorsal hollow nerve cord, pharyngeal gill slits, and a post-anal tail. Urochordata (Ascidia, Salpa) retain the notochord only in the larval tail; Cephalochordata (Branchiostoma/Amphioxus) retain it throughout life. In Vertebrata, the notochord is replaced by a vertebral column.

Vertebrate classes progress in complexity: Cyclostomata (jawless, circular sucking mouth — Petromyzon/lamprey, Myxine/hagfish), Chondrichthyes (cartilaginous skeleton, placoid scales, no air bladder — Scoliodon/shark, Torpedo/electric ray, Pristis/sawfish), Osteichthyes (bony skeleton, swim bladder, operculum — Labeo/rohu, Hippocampus/seahorse, Exocoetus/flying fish), Amphibia (3-chambered heart, moist skin for cutaneous respiration — Bufo/toad, Rana/frog, Ichthyophis/limbless amphibian), Reptilia (3-chambered heart with an incomplete septum — 4-chambered in Crocodilia; dry scaly skin, internal fertilization — Chelone/turtle, Naja/cobra, Crocodilus), Aves (4-chambered heart, feathers, pneumatic bones, air sacs, warm-blooded — Struthio/ostrich, Aptenodytes/penguin, Pavo/peacock), and Mammalia (4-chambered heart, mammary glands, hair, mostly viviparous — except Ornithorhynchus/platypus which is oviparous; Macropus/kangaroo, Balaenoptera/blue whale, Pteropus/flying fox).

The key testable concept is the classification basis (symmetry, germ layers, coelom type), the unique identifying features of each phylum, and key exceptions (crocodile with 4-chambered heart, platypus as egg-laying mammal, Cephalopoda with closed circulation).