Animal Kingdom

1. Basis of Classification

1. Level of Organisation

• Cellular Level: Cells are loosely packed; showing some division of labour. Example: Sponges.
• Tissue Level: Group of cells performing a common function. Examples: Cnidaria, Ctenophora.
• Organ Level: Group of tissues arranged to form organs. Examples: Platyhelminthes & higher phyla.
• Organ System Level: Organs join to form functional systems. Examples: Annelids, Arthropods, Molluscs, Echinoderms, Chordates.

2. Digestive System

• Incomplete System: Single opening acting as both mouth and anus. Example: Platyhelminthes.
• Complete System: Two separate openings (Mouth and Anus).
  • Protostomous: Mouth develops before anus.
  • Deuterostomous: Anus develops before mouth.

3. Circulatory System

• Open System: Blood bathes organs directly; flows through open cavities (sinuses). Examples: Arthropoda, Non-cephalopod Molluscs.
• Closed System: Blood circulates through closed vessels (arteries, veins, capillaries). Example: Annelida (first phylum to show closed system).

4. Body Symmetry

• Asymmetrical: Any plane passing through the center does not divide the body into equal halves. Example: Most Sponges.
• Radial Symmetry: Any plane passing through the central axis divides the body into two identical halves. Examples: Cnidaria, Ctenophora, Adult Echinoderms.
• Bilateral Symmetry: Body can be divided into identical left and right halves in only one plane. Examples: Platyhelminthes, Annelida, Arthropoda, Larval Echinoderms.

5. Embryonic Germ Layers

• Diploblastic Animals: Embryo has 2 germinal layers — Ectoderm & Endoderm, with an undifferentiated jelly-like layer (Mesoglea) in between. Examples: Cnidaria, Ctenophora.
• Triploblastic Animals: Embryo has 3 germinal layers — Ectoderm, Mesoderm & Endoderm. Examples: Platyhelminthes to Chordates.

6. Coelom (Body Cavity)

Body cavity lined by mesoderm between body wall and gut wall.

• Acoelomates: Body cavity absent. Example: Platyhelminthes.
• Pseudocoelomates: Body cavity not fully lined by mesoderm; mesoderm is present as scattered pouches. Example: Aschelminthes.
• Coelomates (Eucoelomates): True body cavity lined fully by mesoderm.
  • Schizocoelom: Formed by splitting of mesoderm. Examples: Annelida, Arthropoda, Mollusca.
  • Enterocoelom: Formed from outpocketing of embryonic gut (archenteron). Examples: Echinodermata, Chordata.

2. Segmentation, Notochord & Non-Chordates

7. Segmentation

Body divided externally and internally into repeated segments showing serial repetition of organs (Metamerism).

• Examples: Annelida, Arthropoda, Chordata.

8. Notochord

Mesodermally derived rod-like structure formed on the dorsal side during embryonic development.

• Chordates: Animals with a notochord (e.g., Chordata).
• Non-Chordates: Animals without a notochord (Porifera to Hemichordata).

3. Survey of Non-Chordates

1. Phylum Porifera (Sponges)

• Commonly known as sponges; primitive multicellular, marine, mostly asymmetrical with cellular level organisation.
• Water Canal System: Water enters through minute pores (Ostia) into central cavity (Spongocoel) and goes out through Osculum. Helps in food gathering, gas exchange, and waste removal.
• Choanocytes (Collar Cells): Special flagellated cells lining spongocoel and canals; maintain water current and trap food particles.
• Skeleton: Made of spicules or spongin fibers.
• Reproduction: Hermaphrodite (bisexual); Sexual by gametes (Internal fertilization, indirect development); Asexual by fragmentation/gemmules.
• Examples: Sycon (Scypha), Euspongia (Bath sponge), Spongilla (Freshwater sponge).

2. Phylum Cnidaria (Coelenterata)

• Aquatic, mostly marine, sessile or free-swimming, radial symmetry, diploblastic, tissue level of organization.
• Central gastrovascular cavity with a single mouth opening on Hypostome. Digestion is both extracellular and intracellular.
• Two Basic Body Forms:
  • Polyp: Cylindrical, sessile, asexual form (e.g., Hydra, Adamsia).
  • Medusa: Umbrella-shaped, free-swimming, sexual form (e.g., Aurelia / Jellyfish).
• Metagenesis: Alternation of generation where Polyp produces Medusa asexually, and Medusa produces Polyp sexually (e.g., Obelia).
• Skeleton in some species is composed of Calcium Carbonate ($CaCO_3$).
• Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea pen), Gorgonia (Sea fan), Meandrina (Brain coral).

3. Phylum Ctenophora (Comb Jellies)

• Exclusively marine, radially symmetrical, diploblastic, tissue level organisation.
• Body bears eight external rows of ciliated comb plates which help in locomotion.
• Shows Bioluminescence (the property of a living organism to emit light).
• Hermaphrodite; reproduces exclusively by sexual means; external fertilization with indirect development.
• Examples: Pleurobrachia (Sea walnut), Ctenoplana.

4. Non-Chordates

4. Phylum Platyhelminthes (Flatworms)

• Dorso-ventrally flattened body; organ level of organisation, bilateral symmetry, triploblastic, acoelomate.
• Mostly endoparasites in human and animals; possess hooks and suckers for anchorage and nutrient absorption.
• Flame Cells (Protonephridia): Specialized cells for osmoregulation and excretion.
• Hermaphrodites; internal fertilization; development through many larval stages; high regeneration capacity (e.g., Planaria).
• Examples: Taenia (Tapeworm), Fasciola (Liver fluke).

5. Phylum Aschelminthes (Roundworms)

• Body circular in cross-section; organ-system level, bilateral symmetry, triploblastic, pseudocoelomate.
• Free-living (aquatic/terrestrial) or parasitic in plants and animals.
• Complete digestive tract with a well-developed muscular pharynx. Excretory tube removes waste through excretory pore.
• Dioecious (sexes separate); females are usually longer than males; internal fertilization.
• Examples: Ascaris (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm).

6. Phylum Annelida (Segmented Worms)

• Aquatic or terrestrial; organ-system level, bilateral symmetry, triploblastic, true coelomate, metamerically segmented.
• Possess longitudinal and circular muscles for locomotion. Aquatic annelids (like Nereis) possess lateral appendages called Parapodia for swimming.
• Closed circulatory system; Nephridia present for osmoregulation and excretion.
• Neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.
• Nereis is dioecious; Earthworms and Leeches are monoecious (hermaphrodite).
• Examples: Nereis, Pheretima (Earthworm), Hirudinaria (Blood-sucking leech).

7. Phylum Arthropoda (Jointed Appendages)

• Largest Phylum of Animalia (covers over 2/3rd of all named species).
• Organ-system level, bilateral symmetry, triploblastic, coelomate, segmented with chitinous exoskeleton. Body consists of Head, Thorax, and Abdomen.
• Respiratory Organs: Gills (prawns), Book gills (king crab/Limulus), Book lungs (scorpions/spiders), Tracheal system (insects).
• Open circulatory system; Sensory organs like antennae, simple/compound eyes, statocysts (balance organs).
• Excretion via Malpighian tubules or green glands. Mostly dioecious, oviparous, internal fertilization.
• Examples: Economical (Apis, Bombyx, Laccifer); Vectors (Anopheles, Culex, Aedes); Living Fossil (Limulus).

5. Non-Chordates & Hemichordata

8. Phylum Mollusca (Soft-Bodied Animals)

• Second largest animal phylum. Terrestrial or aquatic; organ-system level, bilateral symmetry, triploblastic, coelomate.
• Unsegmented body covered by a calcareous shell ($CaCO_3$); distinct Head, Muscular Foot, and Visceral Hump.
• Mantle: Soft skin layer over visceral hump. Mantle cavity contains feather-like gills (Ctenidia) for respiration and excretion.
• Mouth contains a file-like rasping organ for feeding called Radula.
• Usually dioecious, oviparous, indirect development.
• Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea hare).

9. Phylum Echinodermata (Spiny-Skinned)

• Endoskeleton of calcareous ossicles (spiny skinned). All are marine.
• Symmetry Exception: Adult echinoderms are Radially symmetrical, but larvae are Bilaterally symmetrical.
• Triploblastic, coelomate with complete digestive system (Mouth on ventral side, Anus on dorsal side).
• Water Vascular System: Distinct feature that helps in locomotion, capture and transport of food, and respiration. Excretory system is absent.
• Dioecious; external fertilization; free-swimming ciliated larva.
• Examples: Asterias (Starfish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber), Ophiura (Brittle star).

10. Phylum Hemichordata

• Worm-like marine animals; organ-system level, bilateral symmetry, triploblastic, coelomate.
• Body consists of an anterior Proboscis, a Collar (with rudimentary stomochord), and a long Trunk.
• Open circulatory system; respiration via gills; excretion by Proboscis gland.
• Dioecious, external fertilization, indirect development.
• Examples: Balanoglossus, Saccoglossus.

6. Phylum Chordata Overview

Characterized fundamentally by 4 key features:

1. Presence of a Notochord.
2. Dorsal hollow single Nerve Cord.
3. Paired Pharyngeal Gill Slits.
4. Presence of a Post-Anal Tail.

7. Subphylum Vertebrata & Fishes

Classification of Vertebrata

• Agnatha (Lacks jaw): Class Cyclostomata.
• Gnathostomata (Bears jaw):
  • Superclass Pisces (Bears fins): Chondrichthyes & Osteichthyes.
  • Superclass Tetrapoda (Bears limbs): Amphibia, Reptilia, Aves, Mammalia.

a. Class Cyclostomata (Jawless Fishes)

• Ectoparasites on some fishes; elongated body with 6-15 pairs of gill slits for respiration.
• Sucking and circular mouth without jaws; body lacks scales and paired fins. Cranium and vertebral column are cartilaginous.
• Anadromous Migration: Marine adults migrate to freshwater for spawning, die after spawning, and larvae return to ocean after metamorphosis.
• Examples: Petromyzon (Lamprey), Myxine (Hagfish).

b. Class Chondrichthyes (Cartilaginous Fishes)

• Exclusively marine; streamlined body with cartilaginous endoskeleton; ventral mouth; powerful jaws.
• Separate gill slits without operculum (gill cover). Tough skin with microscopic Placoid scales (teeth are modified placoid scales directed backward).
• Air Bladder Absent: Must swim constantly to avoid sinking.
• Two-chambered heart (1 auricle, 1 ventricle); poikilothermous (cold-blooded). Males bear Claspers on pelvic fins. Viviparous.
• Special Organs: Electric organ (Torpedo), Poison sting (Trygon).
• Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark).

c. Class Osteichthyes (Bony Fishes)

• Both marine and freshwater fishes; bony endoskeleton; mouth terminal.
• Four pairs of gills covered by an Operculum on each side. Skin covered with Cycloid or Ctenoid scales.
• Air Bladder Present: Regulates buoyancy in water.
• Two-chambered heart; cold-blooded; mostly oviparous with direct development.
• Examples: Marine (Exocoetus – flying fish, Hippocampus – sea horse); Freshwater (Labeo – rohu, Catla, Clarias – magur); Aquarium (Betta – fighting fish, Pterophyllum – angel fish).

8. Superclass Tetrapoda

1. Class Amphibia (Dual Life)

• Can live both in aquatic and terrestrial habitats; possess two pairs of limbs. Body divided into Head and Trunk (tail present in some larvae).
• Moist skin without scales; eyes have eyelids; Tympanum represents the ear.
• Respiration by gills (larvae), lungs, and moist skin. Three-chambered heart (2 auricles, 1 ventricle); cold-blooded.
• Alimentary canal, urinary, and reproductive tracts open into a common chamber called Cloaca. Oviparous, indirect development.
• Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibia).

2. Class Reptilia (Creeping/Crawling)

• Mostly terrestrial; body covered by dry and cornified skin with epidermal scales or scutes.
• Tympanum represents ear (no external ear opening). Limbs when present are two pairs.
• Three-chambered heart (Exception: Crocodiles have a 4-chambered heart); cold-blooded.
• Snakes and lizards shed their scales as skin cast. Internal fertilization, oviparous, direct development.
• Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Hemidactylus (Wall lizard), Crocodilus, Alligator, Poisonous snakes (Naja, Bangarus, Vipera).

3. Class Aves (Birds)

• Presence of feathers; beak without teeth; forelimbs modified into wings; hindlimbs adapted for walking, swimming, or perching.
• Skin is dry without glands except oil gland (uropygial gland) at the base of the tail.
• Endoskeleton fully ossified (bony); long bones are hollow with air cavities (Pneumatic bones) to reduce weight for flight.
• Digestive tract has additional chambers: Crop (storage) and Gizzard (mastication).
• Four-chambered heart; Homoiothermous (warm-blooded); respiration by lungs supplemented by air sacs. Oviparous, direct development.
• Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich – flightless), Pavo (Peacock), Aptenodytes (Penguin).

4. Class Mammalia

• Found in diverse habitats; most unique characteristic is presence of milk-producing Mammary Glands to nourish young ones.
• Skin possesses hair; external ears or Pinnae present; different types of teeth present in jaws (Heterodont, Thecodont, Diphyodont).
• Four-chambered heart; warm-blooded; respiration by lungs.
• Sexes separate, internal fertilization, mostly Viviparous (Exception: Ornithorhynchus / Duck-billed platypus is Oviparous).
• Examples: Oviparous (Ornithorhynchus, Echidna); Viviparous (Macropus – Kangaroo, Pteropus – Flying fox, Camelus, Macaca, Rattus, Canis, Felis, Elephas, Equus, Delphinus, Balaenoptera – Blue whale, Panthera tigris, Panthera leo).