Basis of classification
Animals look very different, yet a few body-plan features let us sort them.
Levels of organisation
- Cellular: cells in loose groups (sponges).
- Tissue: cells doing the same job form tissues (coelenterates, ctenophores).
- Organ: tissues form organs (flatworms).
- Organ system: organs work together (annelids to chordates). The gut may be incomplete (one opening) or complete (mouth and anus). Blood circulation may be open (blood in open spaces) or closed (blood in vessels).
Symmetry
- Asymmetrical: no plane divides the body into equal halves (most sponges).
- Radial: any plane through the centre gives equal halves (coelenterates, ctenophores, adult echinoderms).
- Bilateral: only one plane gives left and right halves (annelids, arthropods, chordates).
Germ layers, coelom, segmentation and notochord
- Diploblastic: two embryonic layers, ectoderm and endoderm, with a jelly-like mesoglea between (coelenterates). Triploblastic: a third layer, mesoderm, in the middle (flatworms to chordates).
- Coelom = body cavity lined by mesoderm. Acoelomates have none (flatworms). Pseudocoelomates have a cavity but mesoderm is only in scattered pouches (roundworms). Coelomates have a true coelom (annelids, molluscs, arthropods, echinoderms, hemichordates, chordates).
- Metameric segmentation: the body is divided into repeating segments, as in the earthworm.
- Notochord: a stiff rod on the back side, formed from mesoderm in the embryo. Animals with it are chordates; without it, non-chordates.
Non-chordates: phylum by phylum (part 1)
1. Porifera (sponges)
Mostly marine, asymmetrical, cellular level. Water enters through tiny pores (ostia) into a canal system and leaves through the osculum. Collar cells (choanocytes) line the canals. Skeleton of spicules or spongin fibres. Hermaphrodite; fertilisation internal, development indirect. E.g. Sycon, Spongilla (freshwater), Euspongia (bath sponge).
2. Coelenterata (Cnidaria)
Aquatic, radial, diploblastic, tissue level. Have cnidoblasts (stinging cells) on tentacles for defence and catching prey. One opening (hypostome) to a gut cavity. Two body forms: polyp (fixed, like Hydra) and medusa (free-swimming, like Aurelia); some alternate between them (metagenesis, as in Obelia). Corals have a calcium carbonate skeleton. E.g. Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula, Gorgonia, Meandrina.
3. Ctenophora (comb jellies)
Marine, radial, diploblastic. Eight external rows of ciliated comb plates for swimming. Bioluminescence (they glow) is common. E.g. Pleurobrachia, Ctenoplana.
4. Platyhelminthes (flatworms)
Flattened top to bottom, bilateral, triploblastic, acoelomate, organ level. Many are parasites with hooks and suckers. Flame cells help in excretion. Planaria can regrow lost parts. E.g. Taenia (tapeworm), Fasciola (liver fluke).
5. Aschelminthes (roundworms)
Round in cross-section, bilateral, triploblastic, pseudocoelomate. Complete gut with a muscular pharynx. Sexes separate; female longer than male. E.g. Ascaris (roundworm), Wuchereria (filaria worm), Ancylostoma (hookworm).
Non-chordates: phylum by phylum (part 2)
6. Annelida
Bilateral, triploblastic, coelomate, with metameric segmentation (body looks like rings). Closed circulation. Nephridia for excretion. Move with muscles and bristles (setae) or side flaps (parapodia). E.g. Nereis, Pheretima (earthworm), Hirudinaria (leech).
7. Arthropoda
The largest phylum (about two-thirds of all named species). Jointed legs and a hard chitin exoskeleton; body in head, thorax and abdomen. Open circulation. Breathing by gills, book gills, book lungs or tracheae; excretion by Malpighian tubules. E.g. economic: Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect); vectors: Anopheles, Culex, Aedes; pest: Locusta; living fossil: Limulus (king crab).
8. Mollusca
Second largest phylum. Soft body covered by a calcium carbonate shell; body in head, muscular foot and visceral hump; a soft skin fold, the mantle. A rasping tongue, the radula, in the mouth. E.g. Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus.
9. Echinodermata
All marine; spiny calcareous skeleton (echinos = spiny). Adults radial, larvae bilateral. Unique water vascular system with tube feet for moving, catching food and breathing. No excretory system. E.g. Asterias (starfish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Ophiura (brittle star).
10. Hemichordata
Worm-like marine animals. Body in proboscis, collar and trunk. Have a small stomochord in the collar, once thought to be a notochord, so they were earlier grouped with chordates. Breathe by gills. E.g. Balanoglossus, Saccoglossus.
Chordata: features and subphyla
Three basic features appear at some stage of life:
- Notochord: a stiff rod on the back side.
- Dorsal, hollow, single nerve cord.
- Pharyngeal gill slits.
They also have a heart on the belly (ventral) side and a tail behind the anus.
| Chordates | Non-chordates |
|---|---|
| Notochord present | Notochord absent |
| Nerve cord dorsal, hollow, single | Nerve cord ventral, solid, double |
| Gill slits present | Gill slits absent |
| Heart ventral | Heart dorsal (or absent) |
| Post-anal tail present | Post-anal tail absent |
Subphyla
- Urochordata: notochord only in the larval tail, e.g. Ascidia, Salpa, Doliolum.
- Cephalochordata: notochord from head to tail for life, e.g. Branchiostoma (Amphioxus).
- Vertebrata: notochord in the embryo is replaced by a backbone (vertebral column) in the adult. Every vertebrate is a chordate, but not every chordate is a vertebrate.
Vertebrate classes: fishes and amphibians
Cyclostomata
Jawless (Agnatha), sucking round mouth, 6–15 pairs of gill slits, no scales or paired fins, cartilage skeleton. Marine but migrate to fresh water to breed, then die. E.g. Petromyzon (lamprey), Myxine (hagfish).
Chondrichthyes (cartilaginous fish)
Marine, jaws, skeleton of cartilage, mouth on the underside, gill slits not covered (no operculum), tiny placoid scales, teeth are modified scales. No air bladder, so they must keep swimming to avoid sinking. Some have electric organs (Torpedo) or poison stings (Trygon). E.g. Scoliodon (dogfish), Pristis (sawfish), Carcharodon (great white shark).
Osteichthyes (bony fish)
Marine and freshwater, bony skeleton, gills covered by an operculum, mouth at the front, cycloid or ctenoid scales, an air bladder to float. E.g. marine: Exocoetus (flying fish), Hippocampus (sea horse); freshwater: Labeo (rohu), Catla, Clarias (magur); aquarium: Betta, Pterophyllum (angelfish).
Amphibia
Live in water and on land; two pairs of limbs; moist skin without scales; eardrum (tympanum) outside; cloaca (common opening); breathe by gills, lungs and skin; three-chambered heart; cold-blooded (poikilotherms). E.g. Rana (frog), Bufo (toad), Hyla (tree frog), Salamandra, Ichthyophis (limbless).
Vertebrate classes: reptiles, birds and mammals
Reptilia
Mostly land animals that creep or crawl; dry skin with horny scales or scutes; no external ear opening; usually three-chambered heart but four in crocodiles; cold-blooded; lay shelled eggs on land. Snakes and lizards shed skin. E.g. Chelone (turtle), Testudo (tortoise), Chameleon, Calotes (garden lizard), Crocodilus, Naja (cobra), Vipera.
Aves (birds)
Feathers; beak; forelimbs turned into wings; hind limbs with scales; no skin glands except an oil gland at the tail; hollow pneumatic bones for light weight; crop and gizzard in the gut; air sacs help lungs; four-chambered heart; warm-blooded (homeotherms). E.g. Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich), Pavo (peacock), Aptenodytes (penguin).
Mammalia
Mammary glands make milk for the young; body hair; external ears (pinnae); different kinds of teeth; four-chambered heart; warm-blooded; mostly give birth to live young (viviparous), except the egg-laying platypus. E.g. Ornithorhynchus (platypus, lays eggs), Macropus (kangaroo), Pteropus (flying fox), Camelus, Macaca (monkey), Rattus, Canis (dog), Felis (cat), Elephas, Equus (horse), Delphinus (dolphin), Balaenoptera (blue whale), Panthera tigris, Panthera leo.
Try it: sort the animals at home
Make a list of 10 animals you saw this week: ant, earthworm, snail, pigeon, lizard, frog, fish, cow, spider, mosquito. For each, ask: backbone? segments? jointed legs? shell? feathers? hair? Write its phylum or class. Then open the 3D free-play step, pick the same animals and check your answers.
Key formulas and definitions
- Levels: cellular → tissue → organ → organ system
- Symmetry: asymmetrical (sponges) · radial (cnidarians, ctenophores, adult echinoderms) · bilateral (rest)
- Diploblastic (cnidarians, ctenophores) vs triploblastic (flatworms onwards)
- Acoelomate (flatworms) · pseudocoelomate (roundworms) · coelomate (annelids onwards)
- Chordate trio: notochord + dorsal hollow nerve cord + pharyngeal gill slits
- Vertebrate classes: Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, Mammalia
Worked examples
1. An animal has stinging cells on tentacles and one opening to its gut. Name the phylum and an example.
Coelenterata (Cnidaria). Example: Hydra or jellyfish Aurelia.
2. Why is a roundworm called pseudocoelomate?
It has a body cavity, but the mesoderm lies only as scattered pouches between ectoderm and endoderm, not lining the whole cavity. So it is a false coelom.
3. Which phylum has a water vascular system? Give its use.
Echinodermata. The water pipes and tube feet help in moving, catching food and breathing.
4. A shark keeps swimming even while resting, but a rohu can stay still. Why?
A shark (Chondrichthyes) has no air bladder, so it sinks if it stops. A rohu (Osteichthyes) has an air bladder that keeps it afloat.
5. Platypus lays eggs. Why is it still a mammal?
It has hair and mammary glands that make milk for its young, the key features of Mammalia.
6. Classify: earthworm, cockroach, octopus, starfish, Balanoglossus.
Earthworm: Annelida. Cockroach: Arthropoda. Octopus: Mollusca. Starfish: Echinodermata. Balanoglossus: Hemichordata.
Common mistakes
- Calling the starfish a fish. It is an echinoderm; the jellyfish is a cnidarian and the silverfish is an insect.
- Saying hemichordates are chordates. Their stomochord is not a true notochord.
- Writing that all chordates have a backbone. Urochordates and cephalochordates do not; only vertebrates do.
- Saying all reptiles have a three-chambered heart. Crocodiles have four chambers.