Animal Classification: Basis and Organization

• Over a million animal species exist
• Classification is needed for organization and assigning systematic positions

Basis of Classification

• Despite structural differences, fundamental features are shared across animals.
• Arrangement of cells, body symmetry, coelom nature, and digestive, circulatory, and reproductive patterns serve as classification criteria.

Levels of Organization

• Animalia members are multicellular but don't share the same cell organization patterns.
• Sponges: Cells are loosely arranged, showing cellular-level organization with some division of labor.
• Coelenterates: More complex arrangement where cells doing the same job form tissues, resulting in tissue-level organization.
• Platyhelminthes: Exhibit organ-level organization where tissues form specialized organs.
• Annelids, Arthropods, Molluscs, Echinoderms, and Chordates: Organs form functional systems, known as the organ system level of organization.
• Platyhelminthes digestive system: Has a single opening for both mouth and anus = incomplete.
• Complete digestive system: Has two openings - mouth and anus.
• Open circulatory system: Blood is pumped from the heart, directly bathing cells and tissues.
• Closed circulatory system: Blood circulates via vessels of varying diameters (arteries, veins, capillaries).

Symmetry

• Animals are categorized by symmetry.
• Sponges: Mostly asymmetrical, no plane divides them into equal halves.
• Radial symmetry: Body divided into identical halves by any plane passing through the central axis.
• Radial symmetry examples: Coelenterates, ctenophores, and echinoderms.
• Bilateral symmetry: Body divided into identical left and right halves by only one plane.
• Bilateral symmetry examples: Annelids, arthropods.

Diploblastic and Triploblastic Organization

• Diploblastic animals: Cells arranged in two embryonic layers (ectoderm and endoderm).
• Diploblastic animals example: Coelenterates
• Mesoglea: Undifferentiated layer between the ectoderm and endoderm in diploblastic animals.
• Triploblastic animals: Have a third germinal layer, mesoderm, between the ectoderm and endoderm. -Triploblastic animals examples: Platyhelminthes to chordates.

Coelom

• Classification depends on the presence or absence of a cavity between the body and gut walls.
• Coelom: Body cavity lined by mesoderm.
• Coelomate examples: Annelids, molluscs, arthropods, echinoderms, hemichordates, and chordates.
• Pseudocoelom: Body cavity not lined by mesoderm; mesoderm is present in scattered pouches.
• Pseudocoelomate examples: Aschelminthes.
• Acoelomate: Animals lacking a body cavity.
• Acoelomate examples: Platyhelminthes.

Segmentation

• Some animals have bodies divided into segments externally and internally.
• Metameric segmentation: Serial repetition of some organs along the segments.
• Metamerism example: Earthworm.

Notochord

• Notochord: Mesodermally derived rod-like structure on the dorsal side during embryonic development.
• Chordates: Animals possessing a notochord.
• Non-chordates: Animals lacking a notochord.

Classification of Animals

Phylum – Porifera

• Commonly known as sponges.
• Mostly marine and asymmetrical.
• Primitive multicellular organisms exhibiting cellular-level organization.
• Water transport/canal system: Water enters through ostia (minute pores), flows into spongocoel (central cavity), and exits through osculum.
• Water transport functions: Food gathering, respiratory exchange, and waste removal.
• Choanocytes/collar cells: Line the spongocoel and canals.
• Intracellular digestion.
• Skeleton: Supported by spicules or spongin fibres.
• Hermaphrodite: Sexes are not separate; both eggs and sperm are produced by the same individual.
• Asexual reproduction via fragmentation; sexual reproduction via gamete formation.
• Internal fertilization.
• Indirect development with a morphologically distinct larval stage.
• Examples: Sycon (Scypha), Spongilla (freshwater sponge), and Euspongia (bath sponge).

Phylum – Coelenterata (Cnidaria)

• Aquatic (mostly marine), sessile or free-swimming, radially symmetrical.
• Cnidaria is named for cnidoblasts/cnidocytes, which contain stinging capsules/nematocysts.
• Cnidoblasts: Used for anchorage, defense, and prey capture.
• Tissue-level organization, diploblastic.
• Gastro-vascular cavity with a single opening (mouth) on hypostome.
• Digestion is both extracellular and intracellular.
• Corals have a skeleton made of calcium carbonate.
• Two basic body forms: polyp and medusa.
• Polyp: Sessile and cylindrical (e.g., Hydra, Adamsia).
• Medusa: Umbrella-shaped and free-swimming (e.g., Aurelia/jelly fish)
• Metagenesis: Alternation of generation where polyps produce medusae asexually, and medusae form polyps sexually (e.g., Obelia).
• Examples: Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea-pen), Gorgonia (sea-fan), and Meandrina (brain coral).

Phylum – Ctenophora

• Known as sea walnuts or comb jellies.
• Exclusively marine, radially symmetrical, diploblastic.
• Tissue-level organization.
• Eight external rows of ciliated comb plates aid in locomotion.
• Bioluminescence: Property of emitting light.
• Extracellular and intracellular digestion.
• Sexes are not separate. Reproduction primarily by means.
• External fertilization with indirect development.
• Examples: Pleurobrachia and Ctenoplana.

Phylum – Platyhelminthes

• Dorso-ventrally flattened bodies, known as flatworms
• Mostly endoparasites, including those in humans.
• Bilaterally symmetrical, triploblastic, and acoelomate animals.
• Organ-level organization.
• Hooks and suckers are present in parasitic forms.
• Nutrient absorption : Directly through the body surface
• Flame cells: Specialized cells for osmoregulation and excretion.
• Sexes are not separate.
• Internal fertilization and development through many larval stages.
• Planaria: Exhibits high regeneration capacity.
• Examples: Taenia (tapeworm) and Fasciola (liver fluke).

Phylum – Aschelminthes

• Body circular in cross-section, hence the name roundworms.
• Free-living, aquatic, terrestrial, or parasitic in plants and animals.
• Organ-system level of body organization.
• Bilaterally symmetrical, triploblastic, and pseudocoelomate animals.
• Complete alimentary canal with a muscular pharynx.
• Excretory tube removes body wastes through an excretory pore.
• Dioecious: Sexes are separate, with distinct males and females.
• Internal fertilization and development may be direct or indirect.
• Examples: Ascaris (roundworm), Wuchereria (filaria worm), and Ancylostoma (hookworm).

Phylum – Annelida

• Aquatic (marine and fresh water) or terrestrial, free-living, and sometimes parasitic.
• Organ-system level of body organization and bilateral symmetry.
• Triploblastic, metamerically segmented, and coelomate animals.
• Body surface distinctly marked into segments or metameres.
• Longitudinal and circular muscles aid in locomotion.
• Parapodia: Lateral appendages in aquatic annelids like Nereis help in swimming.
• Closed circulatory system.
• Nephridia: Help in osmoregulation and excretion.
• Neural system: Paired ganglia connected by lateral nerves to a double ventral nerve cord.
• Nereis: Dioecious, aquatic.
• Earthworms and Leeches: Monoecious.
• Sexual reproduction.
• Examples: Nereis, Pheretima (earthworm), and Hirudinaria (blood-sucking leech).

Phylum – Arthropoda

• Largest phylum of Animalia, including insects.
• Over two-thirds of all named species on Earth are arthropods.
• Organ-system level of organization.
• Bilaterally symmetrical, triploblastic, segmented, and coelomate animals.
• Chitinous exoskeleton covering the body.
• Body consists of head, thorax, and abdomen.
• Jointed appendages (arthros-joint, poda-appendages).
• Respiratory organs: Gills, book gills, book lungs, or tracheal system.
• Open circulatory system type.
• Sensory organs: Antennae, eyes(compound and simple), and statocysts.
• Malpighian tubules take part in excretion.
• Mostly dioecious.
• Fertilization is usually internal.
• Mostly oviparous.
• Direct or indirect development.
• Examples: Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect), Anopheles, culex and Aedes(Mosquitoes), Locusta(Locust), and living fossil- Limulus(King crab).

Phylum – Mollusca

• Second largest animal phylum.
• Terrestrial or Organ system level of marine or fresh water aquatic habitat.
• Bilaterally symmetrical, triploblastic, and coelomate animals.
• Calcareous shell covered the unsegmented body with a distinct head, muscular foot, and visceral hump.
• Mantle : A soft, spongy layer of muscle that forms over the visceral hump.
• Mantle cavity : The space between the hump and the mantle contains the feather-like gills.
• Sensory tentacles: Present in the region of the anterior head.
• Radula : A file like rasping organ for feeding located in the mouth.
• Usually dioecious an oviparous with indirect development.
• Example : Pila(Apple snail), Pinctada(Pearl oyster), Sepia(cuttlefish), Loligo(Squid), Octopus(Devil fish), Aplysia(Sea-Hare), Denalium(tusk shell) and Chaetopleura(Chiton).

Phylum – Echinodermata

• These animals have an endoskeleton of calcareous ossicles
• Echinodermata named from its spiny body.
• All marine organization with organ system level.
• Radially symmetrical adult echinoderms but bilaterally symmetrical larvae.
• Digestive system is complete with the mouth on the lower(ventral) side and the anus on the upper(dorsal) side.
• distinctive water vascular system helps in transport, respiration capture , food and locomotion.
• Sexes are separate where fertilization is usually external.
• Free-swimming larva with free-swimming larva and indirect development.
• Example : Asterias(star fish), Echinus( sea urchin), Antedon(sea lilly), cucumaria (sea cucumber) and Ophiura (brittle star).

Phylum – Hemichordata

• Earlier considered to be a sub-phylum under the phylum chordata but is now placed as a separate phylum
• Hemichordates have a rudimentary structure called stomochord same as notochord structure in region of collar.
• Small group of marine worms possessing organ system level system.
• Bilerally symmetrical , coelomate and triplioblastic.
• Body is composed long trunk cylinder that carries an anterior proboscis and a collar.
• Open type circulatory system and gills for repiration.
• Proboscis glands that facilitate separate fertilization along with indirect development.
• Example Balanoglossus , Saccoglossus

Phylum – Chordata

• Distinguished as the presence of notochord, with a dorsal
• Characteristics hollow gill slits and bilateral system that is observed with early embryonic life
• Bilaterally symmetrical,triploblastic organisation with closed circulatory
• Table 4.1 presents salient features of chordates
• Chordata phylum devided into 3 subphyla Urochordata or Tunicata ,Vertebrata and Cephalochordata
• Vertebrata and urochordata referred to as Protochordates exclusive only to marine
• Present Vertebrata with persistent in region of thier head to tail

Class – Cyclostomata

• Members has ectoparasitical body and cylindrical with out jaws.
• 6-15 pairs of gill slits that helps them to have a elongated body
• Scales absent and no paired fins.
• Vertebral column and cranium are cartilaginous in structure.
• Circulation is of closed type with migration aquatic.
• Example - sea water for spawning
• Petromyzon - lamprey
• Myxine - hagfish

Class – Chondrichthyes

• The animals are streamlined along with body marine cartilaginous endoskeleton.
• Mouth is located ventrally.
• Throughout the life the Notochord is persistent
• Skin is difficult containing minute
• Modified teeth scales are placoid
• These organisms are predaceous with jaws are powerful
• Air bladder are absent sinking is avoided by swimming constantly

Class – Osteichthyes

• Marine and fresh bony bony endoskeleton for both
• streamlined for body mouth is terminal end is where it is found
• Both sides covered operculum with gills covered
• the presence of air that manages buoyancy and cycle skinoids are covered
• Heart chambered in two coldblooded animal separates
• direct external and mostly development mostly

Class – Amphibia

• Dual life Gr in the water as in the amphibians also .
• Present trunk in head body divides to .
• A typanum depicts eyelid eyes moistly without scales.
• respiratory Urinary canal in elementary opening to extereor ,gills, lungs skin
• 2 Auricies and one ventricle chambers chambered for heart .
• Eggs layed externally that lead to indirect seperate and cold blooed

Class – Reptilia

• Named from it being crawling with creepy or motile .
• Epiderma with skin cornified covered with mostly terrestrial
• Present is the ear not external openings but tympaniam
• Paired limbs or a heart for with chambers with poikilotherms scales .
• Snakes shed with reptile with separate inner of sexual

Class – Aves

• Most characteristic feather feature is the presence birds fly from possessing limbs .
• Forelimbs with wings possess scales and the modified swimming or clasping scales and limbs.
• Hairy for dryness and the gland oil, skeleton air of long
• chambers with an air with of pneumatic . Gizzard digestive heart that the tract birds and 4 that completely .
• warm-blooded animals that are Homoiothermous that maintain the temperature .By Respiration

Class – Mammalia

• Mammallia have a large variety of habitats found in mammals and grasslands .
• They live in water with glands and has a unique characteristics that nourishes mammals.
  • adapted for running and walking and two limbs paired to the skin
  • External present in ears that possess types of teeth
  • Internal with separate with a type of four
  • Air ways Respiration to lungs and blooded the Warm has homoiothermous
  • Direct exceptions with fewer Table summarises all distinguishing under the phyla of animal