Animal Classification & Organization

• Animals have diverse structures and forms, but share fundamental features like cell arrangement, body symmetry, coelom, digestive, circulatory, and reproductive systems.
• These shared features are the basis for animal classification.

Levels of Organization

• Animalia members are multicellular, but have varying cell organization.
• In sponges, cells form loose aggregates, showing a cellular level of organization with some division of labor.
• Coelenterates have a more complex arrangement where cells performing the same function form tissues, known as tissue level organization.
• Platyhelminthes exhibit organ level organization, where tissues form specialized organs.
• Annelids, Arthropods, Molluscs, Echinoderms, and Chordates have organs organized into functional systems, referred to as organ system level of organization, with varying complexity.
• Platyhelminthes have an incomplete digestive system with a single opening, while complete digestive systems have both mouth and anus.
• The circulatory system can be open (blood bathes tissues directly) or closed (blood circulates through vessels).

Symmetry

• Animals can be categorized based on symmetry.
• Sponges are asymmetrical.
• Radial symmetry occurs when any plane through the central axis divides the organism into identical halves (e.g., Coelenterates, Ctenophores, Echinoderms).
• Bilateral symmetry is when the body can be divided into identical left and right halves by only one plane (e.g., Annelids, Arthropods).

Diploblastic and Triploblastic Organization

• Diploblastic animals have cells arranged in two embryonic layers: ectoderm and endoderm (e.g., coelenterates), with mesoglea in between.
• Triploblastic animals have three germinal layers: mesoderm between ectoderm and endoderm (Platyhelminthes to Chordates).

Coelom

• The presence or absence of a body cavity between the body wall and gut is important for classification.
• Coelomates have a body cavity lined by mesoderm (e.g., Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates, Chordates).
• Pseudocoelomates have a body cavity where mesoderm is present as scattered pouches (e.g., Aschelminthes).
• Acoelomates lack a body cavity (e.g., Platyhelminthes).

Segmentation

• Some animals have bodies divided into segments with serial repetition of organs, known as metameric segmentation (e.g., Earthworm, and the phenomenon is called Metamerism.

Notochord

• Notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development.
• Animals with a notochord are called chordates; those without are non-chordates (e.g., Porifera to Echinoderms).

Classification of Animals

• The basic body plan are used for the classification of animals.

Phylum Porifera

• Sponges are mostly marine and asymmetrical animals with cellular level organization.
• They have a water transport or canal system where water enters through ostia into the spongocoel and exits through the osculum.
• Choanocytes line the spongocoel and canals.
• Digestion is intracellular.
• The skeleton is made of spicules or spongin fibres.
• Sponges are hermaphrodite.
• They reproduce asexually by fragmentation and sexually by gamete formation.
• Fertilization is internal, with indirect development including a larval stage.
• Specific examples include: Sycon (Scypha), Spongilla (freshwater sponge), and Euspongia (bath sponge).

Phylum Coelenterata (Cnidaria)

• Coelenterates are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals.
• Cnidaria have cnidoblasts or cnidocytes used for anchorage, defense, and prey capture.
• They exhibit tissue level organization and are diploblastic, with a central gastro-vascular cavity and a mouth on the hypostome.
• Digestion is extracellular and intracellular.
• Some have skeletons made of calcium carbonate (corals).
• Two basic body forms include polyp (sessile, cylindrical) and medusa (umbrella-shaped, free-swimming).
• Some exhibit alternation of generation (Metagenesis), where polyps produce medusae asexually, and medusae form polyps sexually (e.g., Obelia).
• Specific examples include: Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea-pen), Gorgonia (sea-fan), and Meandrina (brain coral).

Phylum Ctenophora

• Ctenophores, known as sea walnuts or comb jellies, are exclusively marine, radially symmetrical, diploblastic.
• They possess eight external rows of ciliated comb plates for locomotion.
• Digestion is both extracellular and intracellular.
• Bioluminescence is well-marked.
• Sexes are not separate.
• Reproduction is only sexual.
• Fertilization is external with indirect development.
• Specific examples include: Pleurobrachia and Ctenoplana.

Phylum Platyhelminthes

• Platyhelminthes, or flatworms, has a dorso-ventrally flattened body, mostly endoparasites.
• They are bilaterally symmetrical, triploblastic, and acoelomate with organ level organization.
• Parasitic forms possess hooks and suckers.
• Absorb nutrients through their body surface, with specialized flame cells for osmoregulation and excretion.
• Sexes are not separate.
• Fertilization is internal.
• Development occurs through many larval stages.
• Planaria exhibits high regeneration capacity.
• Examples: Taenia (tapeworm) and Fasciola (liver fluke).

Phylum Aschelminthes

• Aschelminthes, or roundworms, have circular bodies in cross-section, may be free-living, aquatic, terrestrial, or parasitic.
• Roundworms are bilaterally symmetrical, triploblastic, and pseudocoelomate with organ-system level of organization.
• The alimentary canal is complete with a muscular pharynx.
• An excretory tube removes body wastes.
• Sexes are separate, where females are often longer than males.
• Fertilization is internal, and development may be direct or indirect.
• Examples: Ascaris (roundworm), Wuchereria (filaria worm), and Ancylostoma (hookworm).

Phylum Annelida

• Annelida can be aquatic (marine or fresh water) or terrestrial, free-living or parasitic.
• Show organ-system level of organization and bilateral symmetry with triploblastic, metamerically segmented and coelomate bodies.
• Possess longitudinal and circular muscles for locomotion.
• Aquatic annelids such as Nereis have parapodia for swimming.
• A closed circulatory system is present.
• Nephridia aid in osmoregulation and excretion.
• The neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.
• Nereis are dioecious, while earthworms and leeches are monoecious.
• Reproduction is sexual.
• Examples: Nereis, Pheretima (earthworm), and Hirudinaria (blood-sucking leech).

Phylum Arthropoda

• Arthropoda is the largest phylum in Animalia, including insects.
• They have organ-system level organization, bilateral symmetry, are triploblastic, segmented, and coelomate.
• The body is covered by a chitinous exoskeleton with components of head, thorax, and abdomen.
• They possess jointed appendages.
• Respiratory organs include gills, book gills, book lungs, or tracheal system.
• An open circulatory system is present.
• Sensory organs include antennae, eyes, and statocysts.
• Excretion occurs through malpighian tubules.
• They are mostly dioecious with internal fertilization and are usually oviparous.
• Development may be direct or indirect.
• Examples include: Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect), Anopheles, Culex, Aedes (mosquitoes), Locusta (locust), and Limulus (king crab).

Phylum Mollusca

• Mollusca is the second largest animal phylum, and includes terrestrial or aquatic animals with organ-system level organization.
• They are bilaterally symmetrical, triploblastic, and coelomate.
• Bodies are covered by a calcareous shell, unsegmented, and consist of a distinct head, muscular foot, and visceral hump.
• A soft, spongy layer of skin forms a mantle over the visceral hump.
• Feather-like gills reside in the mantle cavity, which functions for respiratory and excretory purposes.
• The anterior head region hosts sensory tentacles and the mouth contains a radula.
• Examples: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia (sea-hare), Dentalium (tusk shell), and Chaetopleura (chiton).

Phylum Echinodermata

• Echinodermata possess an endoskeleton of calcareous ossicles.
• All marine, organ-system level organization, adults display radial symmetry, while larvae are bilaterally symmetrical.
• Triploblastic and coelomate animals.
• Digestive systems are complete, with mouths on the lower (ventral) side and anuses on the upper (dorsal) side.
• Posses a water vascular system is used for locomotion, capture and transport of food, and respiration.
• Have separate sexes.
• Usually have sexual reproduction, external fertilization, with indirect development via a swimming larva.
• Examples: Asterias (star fish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), and Ophiura (brittle star).

Phylum Hemichordata

• These were earlier grouped under phylum Chordata.
• These have a rudimentary structure (stomochord) in the collar region, similar to the notochord.
• Worm-like Marine animals with organ-system level organization.
• Animals which are Bilaterally symmetrical, triploblastic and coelomate.
• Cylindrical body includes an anterior proboscis, a collar, and a long trunk.
• An open circulatory system is present.
• Respiration happens with gills.
• The proboscis gland acts as the excretory organ.
• Having separate sexes while fertilization is external and development is indirect.
• Examples: Balanoglossus and Saccoglossus.

Phylum Chordata

• Chordata are characterized by the presence of a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits.
• They are bilaterally symmetrical, triploblastic, coelomate with organ-system level organization with a post anal tail and a closed circulatory system.
• Phylum Chordata is further divided into three subphyla: Urochordata or Tunicata, Cephalochordata, and Vertebrata. Urochordata and Cephalochordata are referred to as Protochordates which are exclusively marine.
• In Urochordata, the notochord is only present in the larval tail whereas in Cephalochordata, it extends from head to tail and lasts throughout life.
• Subphylum Vertebrata possess notochord during the embryonic period which is replaced by a cartilaginous or bony vertebral column in the adult.
• Vertebrates have a ventral muscular heart with two, three, or four chambers, kidneys for excretion and osmoregulation, and paired appendages (fins or limbs).

Class Cyclostomata

• Cyclostomata includes ectoparasites on some fishes and elongated bodies, bearing 6-15 pairs of gill slits for respiration.
• They have a sucking and circular mouth without jaws.
• Their bodies are devoid of scales and paired fins, and the cranium and vertebral column are cartilaginous.
• Circulation is closed type.
• Cyclostomes are marine but migrate for spawning to freshwater.
• Their examples are Petromyzon (Lamprey) and Myxine (Hagfish).

Class Chondrichthyes

• Chondrichthyes includes marine animals with streamlined bodies and cartilaginous endoskeletons.
• The mouth is located ventrally, a notochord is persistent throughout life, and gills slits are separate and do not have an operculum (gill cover).
• The skin is tough and contains minute placoid scales, teeth are modified placoid scales which are backwardly directed, and jaws are very powerful.
• These animals are predaceous, so they must swim constantly to avoid sinking.
• The heart is two-chambered.
• Some may have electric organs (e.g., Torpedo) and poison sting (e.g., Trygon).
• Sexes are separate and have internal fertilization. Many of them are viviparous.
• Examples are Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), and Trygon (Sting ray).

Class Osteichthyes

• Osteichthyes includes marine and freshwater fishes with bony endoskeletons and streamlined bodies.
• The Mouth is mostly terminal, four pairs of gills are covered by an operculum on each side, and skin is covered with cycloid/ctenoid scales.
• An air bladder is present, it regulates buoyancy.
• The Heart is two-chambered, sexes are separate, and fertilization is usually external. They are mostly oviparous and development is direct.
• Examples are Exocoetus (flying fish), Hippocampus (sea horse), Labeo (Rohu), Catla (Katla), Clarias (Magur), Betta (fighting fish), and Pterophyllum (angel fish).

Class Amphibia

• Amphibians live in aquatic and terrestrial habitats with bodies divisible into a head and trunk.
• The skin is moist (without scales), eyes have eyelids, and a tympanum represents the ear.
• The alimentary canal, urinary, and reproductive tracts open into a common chamber called cloaca which opens to the exterior,.
• Respiration happens through gills, lungs, and skin.
• The heart is three-chambered, sexes are separate, and fertilization is external.
• They are oviparous and development is indirect.
• Examples are Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander), and Ichthyophis (limbless amphibia).

Class Reptilia

• Reptilia creep or crawl with dry and cornified skin, epidermal scales or scutes.
• They lack external ear openings, show a tympanum, and limbs, when present, are two pairs.
• The Heart is usually three-chambered, except for crocodiles. Reptiles are poikilotherms.
• Snakes and lizards shed their scales as skin casts. Sexes are separate, they have internal fertilization, and are oviparous with direct development.
• EXamples are Chelone (turtle), Testudo (tortoise), Chameleon (tree lizard), Calotes (garden lizard), Crocodilus (crocodile), Alligator (alligator), Hemidactylus (wall lizard), Naja (cobra), Bangarus (krait), and Vipera (viper).

Class Aves

• Aves possess feathers, and most can fly except flightless birds.
• They possess a beak and forelimbs which are modified into wings.
• The Hind limbs are modified for walking, swimming, or clasping.
• Skin is dry without glands except for the oil gland at the base of the tail.
• The endoskeleton is bony, and long bones are hollow with air cavities (pneumatic).
• The digestive tract has additional chambers (crop and gizzard).
• Hearts are four-chambered.
• Birds are warm-blooded (homoiothermous) and have respiration by the lungs.
• Examples are Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich), Pavo (peacock), Aptenodytes (penguin), and Neophron (vulture).

Class Mammalia

• Mammalia is found in various habitats. The most unique mammalian characteristic is the presence of milk-producing glands mammary glands by which the young ones are nourished.
• Two pairs of limbs adapted for various activities.
• Their skin is unique in possessing hair. External ears or pinnae are present and different types of teeth in the jaw.
• Hearts are four-chambered and homoiothermous.
• Respiration is by lungs, sexes are separate, and fertilization is internal.
• They are viviparous with a few exceptions and development is direct.
• Examples: Ornithorhynchus (platypus), Macropus (kangaroo), Pteropus (flying fox), Camelus (camel), Macaca (monkey), Rattus (rat), Canis (dog), Felis (cat), Elephas (elephant), Equus (horse), Delphinus (common dolphin), Balaenoptera (blue whale), Panthera tigris (tiger), and Panthera leo (lion).