Introduction
Animals are multicellular, eukaryotic organisms that lack cell walls and are primarily heterotrophic (obtaining nutrition by ingestion). The animal kingdom is remarkably diverse — from simple sponges to complex vertebrates. Classification is based on body symmetry, coelom, level of organisation, segmentation, notochord, and phylogenetic relationships.
Basis of Classification
- 1. Levels of Organisation
- Cellular level: cells perform division of labour but no tissues ( · Porifera · )
- Tissue level: cells form tissues ( · Coelenterata · )
- Organ level: tissues form organs ( · Platyhelminthes · )
- Organ-system level: organs form systems ( · Annelida · onwards)
- 2. Symmetry
- Asymmetry: no plane of symmetry ( · Sponges · )
- Radial symmetry: any plane through central axis divides body into equal halves ( · Coelenterata · , · Echinodermata · )
- Bilateral symmetry: only ONE plane divides into equal halves ( · most animals · )
- 3. Diploblastic vs Triploblastic
- Diploblastic: two embryonic layers — ectoderm and endoderm; mesoglea between them ( · Coelenterata · )
- Triploblastic: three layers — ectoderm, mesoderm, endoderm ( · Platyhelminthes · onwards)
- 4. Coelom (body cavity lined with mesoderm)
- Acoelomate: no coelom; solid body ( · Platyhelminthes · )
- Pseudocoelomate: false coelom (not lined with mesoderm) ( · Aschelminthes/Nematoda · )
- Coelomate (Eucoelomate): true coelom ( · Annelida · onwards)
5. Segmentation: Division of body into repetitive units (metameres) — · Annelida · , Arthropoda, Chordata.
6. Notochord: A mesodermal rod-like structure providing support. Present in Phylum Chordata.
Major Animal Phyla
- Porifera (Sponges)
- Cellular level, asymmetrical, no true tissues
- Canal system for water circulation: ostia (pores in) → spongocoel → osculum (out)
- Skeleton of spicules (CaCO3 or SiO2) or spongin fibres
- Examples: · Sycon · , · Spongilla · (freshwater), · Euspongia · (bath sponge)
- Coelenterata / Cnidaria
- Tissue level, radial symmetry, diploblastic
- Cnidoblasts (nematocysts) for prey capture and defence
- Two body forms: polyp (sessile, asexual) and medusa (free-swimming, sexual) — alternation of generations
- Examples: · Hydra · , · Aurelia · (jellyfish), · Adamsia · (sea anemone), · Physalia · (Portuguese man-o-war), Corals
- Platyhelminthes (Flatworms)
- Bilaterally symmetrical, triploblastic, acoelomate
- Flattened body; no anus (incomplete digestive system)
- Many are parasitic: · Taenia · (tapeworm), · Fasciola · (liver fluke), · Schistosoma · (blood fluke)
- Free-living: · Planaria · (high regeneration ability)
- Aschelminthes / Nematoda (Roundworms)
- Bilaterally symmetrical, triploblastic, pseudocoelomate
- Complete digestive system (mouth + anus)
- · Ascaris · (roundworm in humans), · Wuchereria · (filariasis/elephantiasis), · Enterobius · (pinworm)
- Annelida (Segmented Worms)
- Bilaterally symmetrical, triploblastic, true coelom, metameric segmentation
- Closed circulatory system; excretion by nephridia
- · Nereis · (marine), · Pheretima · (earthworm), · Hirudinaria · (leech)
- Earthworm has setae (bristles); leech has suckers
- Arthropoda (Largest animal phylum)
- Exoskeleton of chitin; jointed appendages; open circulatory system
- Excretion by Malpighian tubules (insects) or green glands (crustaceans)
- Examples: · Locusta · (locust), · Apis · (honeybee), · Bombyx · (silkworm), · Anopheles · (mosquito), · Limulus · (king crab — living fossil), · Scorpio ·
- Mollusca (Second largest phylum)
- Soft body; mantle secretes shell of CaCO3; muscular foot
- Open circulatory system (except · Sepia · , · Octopus · — closed)
- · Pila · (apple snail), · Unio · (freshwater mussel), · Pinctada · (pearl oyster), · Sepia · (cuttlefish), · Octopus ·
- Echinodermata
- Marine; radial symmetry (adults), bilateral (larvae); triploblastic, coelomate
- Water vascular system for locomotion and food capture (tube feet)
- Endoskeleton of calcareous ossicles; high regeneration ability
- · Asterias · (starfish), · Echinus · (sea urchin), · Holothuria · (sea cucumber), · Ophiura · (brittle star)
- Chordata
- Three key features: notochord, dorsal hollow nerve cord, pharyngeal gill slits at some stage
- Sub-phyla: Urochordata ( · Ascidia · ), Cephalochordata ( · Amphioxus/Branchiostoma · ), Vertebrata
- Vertebrates: bony/cartilaginous endoskeleton; ventral heart; closed circulation
- Classes: Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, Mammalia
Common mistakes
- Arthropoda is the largest phylum; Mollusca is the second largest.
- Echinoderms show radial symmetry as adults but bilateral symmetry as larvae.
- Sponges (Porifera) are acellular — no tissues — not multicellular at tissue level.
- Coelom is lined by mesoderm; pseudocoelom is NOT lined by mesoderm.
Summary
- Animals are classified by symmetry, embryonic layers, coelom, segmentation, and notochord.
- Porifera → Coelenterata → Platyhelminthes → Aschelminthes → Annelida → Arthropoda → Mollusca → Echinodermata → Chordata represents increasing complexity.
- Arthropoda is the largest phylum; all chordates share notochord, dorsal nerve cord, and pharyngeal gill slits.