Animal Biology: Classification and Diversity
Zoology is the branch of biology that deals with animals. Animalia is one of the five (or six) kingdoms of life — characterised by being multicellular, eukaryotic, heterotrophic organisms that lack cell walls and develop from embryos.
The systematic arrangement of animals into hierarchical groups based on shared characters and evolutionary relationships. The standard ranks are kingdom, phylum, class, order, family, genus and species, codified in the International Code of Zoological Nomenclature (ICZN), latest edition 1999.
Kingdom Animalia — key criteria
- Multicellular with cells lacking cell walls.
- Heterotrophic — ingest food (most), absorb (parasitic), or filter feed.
- Diploid somatic cells (haploid only in gametes).
- Develop from embryos; most form a blastula.
- Lack chloroplasts.
Major animal phyla
Approximately 35 animal phyla are currently recognised; the major ones to know for CSS are:
| Phylum | Approximate species | Key features | Examples |
|---|---|---|---|
| Porifera | 9,000 | Sponges; no true tissues; pores; sessile | Sycon, bath sponge |
| Cnidaria | 11,000 | Radial symmetry; cnidocytes; polyp/medusa | Hydra, jellyfish, coral |
| Platyhelminthes | 25,000 | Flatworms; bilateral; acoelomate | Planaria, tapeworm |
| Nematoda | 25,000+ | Roundworms; pseudocoelomate | Ascaris, hookworm |
| Annelida | 17,000 | Segmented worms; coelom | Earthworm, leech, Nereis |
| Arthropoda | 1.2 million+ | Jointed appendages; exoskeleton of chitin | Insects, spiders, crustaceans |
| Mollusca | 85,000 | Soft body; mantle; many with shells | Snail, clam, octopus |
| Echinodermata | 7,000 | Radial (5) symmetry adult; water vascular system | Starfish, sea urchin |
| Chordata | 65,000 | Notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail | Vertebrates + tunicates, lancelets |
Symmetry and germ layers
- Asymmetry — sponges.
- Radial symmetry — cnidarians, echinoderms (adult).
- Bilateral symmetry — most animals; head-tail axis enables cephalisation.
Germ layers:
- Diploblastic — two layers (ectoderm, endoderm) in cnidarians.
- Triploblastic — three layers (add mesoderm) in all other animals.
Coelom (body cavity):
- Acoelomate — no body cavity (flatworms).
- Pseudocoelomate — cavity between mesoderm and endoderm (nematodes).
- Coelomate — fully lined cavity (annelids onwards).
Chordata in detail
Four diagnostic chordate characters at some life stage:
- Notochord — flexible rod.
- Dorsal hollow nerve cord.
- Pharyngeal slits.
- Post-anal tail.
Subphyla:
- Urochordata (tunicates) — sea squirts; sessile adults.
- Cephalochordata — Branchiostoma (amphioxus).
- Vertebrata — fishes, amphibians, reptiles, birds, mammals.
Classes of Vertebrata
| Class | Examples | Distinguishing features |
|---|---|---|
| Agnatha | Lamprey, hagfish | Jawless |
| Chondrichthyes | Shark, ray | Cartilaginous skeleton |
| Osteichthyes | Bony fishes (Rohu, tuna) | Bony skeleton; gills with operculum |
| Amphibia | Frog, toad, salamander | Dual life; moist skin |
| Reptilia | Lizard, snake, crocodile | Scales; amniotic egg |
| Aves | Birds | Feathers; warm-blooded |
| Mammalia | Mammals | Hair, mammary glands, three middle-ear bones |
- ICZN governs zoological naming (current edition 1999, 4th ed.).
- Arthropoda is the largest phylum (~80% of named animal species).
- Triploblastic, coelomate, bilaterally symmetrical — typical "higher" body plan.
- Mammalian features: hair, mammary glands, three middle-ear bones, single lower jaw bone.
- Echinoderms are deuterostomes (like chordates); arthropods/molluscs are protostomes.
Protostomes vs deuterostomes
- Protostomes — blastopore becomes mouth; spiral cleavage; arthropods, molluscs, annelids.
- Deuterostomes — blastopore becomes anus; radial cleavage; echinoderms, chordates.
Mammalia subclasses
- Prototheria — egg-laying; platypus, echidna.
- Metatheria — pouched; kangaroo, opossum.
- Eutheria — placental; most mammals.
Major eutherian orders: Primates, Carnivora, Rodentia, Cetacea, Chiroptera, Artiodactyla, Perissodactyla.
Animal diversity in Pakistan
Pakistan harbours roughly:
- 195 mammal species including snow leopard, markhor, ibex, urial, Indus river dolphin (Platanista minor), Asiatic black bear.
- 668 bird species — golden eagle, houbara bustard (CMS-listed), Indus migration corridor.
- 177 reptile species — gharial (locally extinct), mugger crocodile.
- 22 amphibians.
- 198 freshwater fish.
Indus River Dolphin (Platanista minor) — endemic to the Indus; declared the national mammal of Pakistan in some sources; ~2,000 individuals remain.
Markhor (Capra falconeri) — national animal.
For zoology MCQs, distinguish acoelomate, pseudocoelomate, coelomate and protostome vs deuterostome. Remember the four chordate hallmarks and that arthropods are the largest phylum. Cite ICZN (1999) as the naming code, parallel to ICN for plants.
Important zoologists
- Aristotle — Historia Animalium; the first classification.
- Carl Linnaeus — Systema Naturae (1758, 10th ed.); starting point for zoological nomenclature.
- Charles Darwin — On the Origin of Species (1859).
- Ernst Mayr — biological species concept (1942).
- Konrad Lorenz, Niko Tinbergen, Karl von Frisch — ethology Nobel 1973.
- E. O. Wilson — sociobiology; ant taxonomy.
Modern molecular phylogeny
Cladistics has rearranged some classical groupings — e.g. birds are recognised as avian theropod dinosaurs, and reptiles in the older sense are paraphyletic. Modern molecular trees use mitochondrial cytochrome b, COI (DNA barcoding standard), and whole-genome data.