Animal Kingdom

Basis of Animal ClassificationThe Major Invertebrate (Non-Chordate) PhylaPhylum Chordata and the Vertebrate Classes

Basis of Animal Classification

The animal kingdom is vast, so animals are classified using a set of fundamental features common across phyla, rather than their outward form. The key criteria are:

  • Levels of organisation — cellular (cells loosely arranged, e.g. sponges), tissue (cells form tissues, e.g. cnidarians), organ, and organ-system level (e.g. higher animals).
  • Symmetry — asymmetrical (no plane divides it into equal halves, e.g. sponges), radial (any plane through the centre gives equal halves, e.g. cnidarians, echinoderms) or bilateral (only one plane gives two equal halves, e.g. most higher animals).
  • Germ layers — diploblastic (two layers: ectoderm and endoderm, e.g. cnidarians) or triploblastic (three layers, with a middle mesoderm, e.g. flatworms onwards).
  • Coelom (body cavity) — acoelomate (no cavity, e.g. flatworms), pseudocoelomate (a false cavity, e.g. roundworms) or coelomate (a true body cavity lined by mesoderm, e.g. annelids onwards).
  • Segmentation — whether the body is divided into repeating segments (metamerism), e.g. earthworm.
  • Notochord — a flexible rod along the back. Animals with a notochord (at some stage) are chordates; those without are non-chordates (invertebrates).

These shared features let biologists group millions of animals into about a dozen major phyla and trace their evolutionary relationships.

1
Worked Example
Example 1: Distinguish radial and bilateral symmetry, with an example of each.
Solution

Symmetry is judged by planes of equal halves.

  • Radial: any plane through the central axis gives equal halves (e.g. a starfish, jellyfish).
  • Bilateral: only one plane gives two equal (mirror) halves (e.g. a human, insect).
2
Worked Example
Example 2: Differentiate diploblastic and triploblastic animals.
Solution

Count the germ layers.

  • Diploblastic: two germ layers (ectoderm and endoderm), e.g. cnidarians.
  • Triploblastic: three layers (with a middle mesoderm), e.g. flatworms and higher animals.
3
Worked Example
Example 3: What is a coelom, and how does an acoelomate differ from a coelomate?
Solution

The coelom is a body cavity.

  • A coelom is a true body cavity lined by mesoderm.
  • Acoelomates have no body cavity (e.g. flatworms); coelomates have a true coelom (e.g. annelids onwards).

Key Points

    • Animals are classified by fundamental features: levels of organisation, symmetry, germ layers, coelom, segmentation and the notochord.
    • Symmetry: asymmetrical / radial / bilateral; germ layers: diploblastic / triploblastic.
    • Coelom: acoelomate / pseudocoelomate / coelomate.
    • Presence of a notochord separates chordates from non-chordates.
✎ Quick Check — 2 questions0 / 2
Q1.An animal that can be divided into two equal halves by only one plane shows ___ symmetry.
Explanation: Bilateral symmetry means only one plane gives two equal (mirror) halves.
Q2.A true body cavity lined by mesoderm is called a:
Explanation: The coelom is a true, mesoderm-lined body cavity.

The Major Invertebrate (Non-Chordate) Phyla

The non-chordates (invertebrates) lack a notochord. The major phyla, roughly in increasing complexity, are:

  • Porifera (sponges) — cellular level; have pores (ostia) and canals; water-canal system; mostly marine. E.g. Sycon, Spongilla.
  • Cnidaria (Coelenterata) — tissue level; radial symmetry, diploblastic; have stinging cells (cnidoblasts). E.g. Hydra, jellyfish, corals, sea anemone.
  • Ctenophora — 'comb jellies'; marine, radially symmetrical, diploblastic; show bioluminescence.
  • Platyhelminthes (flatworms) — bilateral, triploblastic, acoelomate; dorsoventrally flattened; many are parasites. E.g. Planaria, liver fluke, tapeworm.
  • Aschelminthes (roundworms) — bilateral, triploblastic, pseudocoelomate; many are parasites. E.g. Ascaris, Wuchereria (causes filariasis).
  • Annelida — the first segmented, true coelomate animals; bilateral, triploblastic. E.g. earthworm, Nereis, leech.
  • Arthropoda — the largest phylum; jointed appendages, chitinous exoskeleton, open circulation. E.g. insects, prawn, spider, scorpion, Apis (honey bee).
  • Mollusca — the second largest phylum; soft body usually with a calcareous shell. E.g. snail, octopus, Pila, mussel.
  • Echinodermata — marine; spiny skin, water-vascular system, radial symmetry in adults. E.g. starfish, sea urchin.
  • Hemichordata — a small group once placed with chordates; e.g. Balanoglossus.
1
Worked Example
Example 1: Name the largest animal phylum and give two of its features.
Solution

The largest phylum is Arthropoda.

  • Features: jointed appendages and a chitinous exoskeleton.
  • (Also open circulatory system.) Examples include insects and prawns.
2
Worked Example
Example 2: Which phylum is the first to show true segmentation and a true coelom?
Solution

Segmentation and a true coelom first appear together in one phylum.

  • Annelida (e.g. earthworm) is the first segmented, true coelomate phylum.
3
Worked Example
Example 3: Name the special cells of Cnidaria and one example animal.
Solution

Cnidarians have stinging cells.

  • Special cells: cnidoblasts (stinging cells used for defence/capture of prey).
  • Example: Hydra (also jellyfish, corals).

Key Points

    • Porifera (sponges, pores), Cnidaria (cnidoblasts, radial), Ctenophora (comb jellies).
    • Platyhelminthes (flatworms, acoelomate), Aschelminthes (roundworms, pseudocoelomate).
    • Annelida = first segmented true coelomates; Arthropoda = largest phylum (jointed legs, chitin exoskeleton).
    • Mollusca (soft body, shell), Echinodermata (spiny skin, water-vascular system), Hemichordata.
✎ Quick Check — 2 questions0 / 2
Q1.The largest phylum in the animal kingdom is:
Explanation: Arthropoda, with jointed appendages and a chitin exoskeleton, is the largest phylum.
Q2.True segmentation (metamerism) and a true coelom are first seen together in:
Explanation: Annelids (e.g. earthworm) are the first segmented, true coelomate animals.

Phylum Chordata and the Vertebrate Classes

Members of phylum Chordata have, at some stage of life, three defining features: a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits. They are bilaterally symmetrical, triploblastic, coelomate animals. Chordata is divided into three subphyla: Urochordata (e.g. Ascidia), Cephalochordata (e.g. Branchiostoma/Amphioxus) and the largest, Vertebrata.

In Vertebrata, the notochord is replaced (during development) by a vertebral column (backbone), and they have a well-developed brain protected by a cranium. Vertebrates are grouped into five classes:

  • Pisces (fishes) — aquatic; breathe with gills; have fins and scales; cold-blooded; mostly egg-laying. E.g. Labeo (rohu), shark, Catla.
  • Amphibia — live both in water and on land; breathe through gills, lungs and moist skin; cold-blooded; lay eggs in water. E.g. frog, toad, salamander.
  • Reptilia — creeping land animals with dry, scaly skin; breathe by lungs; cold-blooded; lay shelled eggs on land. E.g. lizard, snake, turtle, crocodile.
  • Aves (birds) — body covered with feathers; forelimbs modified into wings; have a beak; warm-blooded; lay eggs. E.g. crow, pigeon, any bird.
  • Mammalia — have hair/fur and mammary glands (feed young on milk); warm-blooded; most give birth to young ones (viviparous). E.g. human, cow, whale, bat.

Birds and mammals are warm-blooded (homeothermic) — they keep a constant body temperature; the other three classes are cold-blooded (poikilothermic).

1
Worked Example
Example 1: State the three fundamental features that define a chordate.
Solution

All chordates share these (at some life stage).

  • A notochord.
  • A dorsal hollow nerve cord.
  • Paired pharyngeal gill slits.
2
Worked Example
Example 2: Which two vertebrate classes are warm-blooded, and what does that mean?
Solution

Warm-blooded animals control their temperature.

  • Aves (birds) and Mammalia are warm-blooded (homeothermic).
  • They keep a constant body temperature regardless of the surroundings.
3
Worked Example
Example 3: Give two features that distinguish mammals from other vertebrates.
Solution

Mammals have unique features.

  • Presence of hair/fur on the body.
  • Presence of mammary glands to feed the young on milk.

Key Points

    • Chordata: notochord + dorsal hollow nerve cord + pharyngeal gill slits (at some stage); subphyla Urochordata, Cephalochordata, Vertebrata.
    • In vertebrates the notochord is replaced by a vertebral column; five classes: Pisces, Amphibia, Reptilia, Aves, Mammalia.
    • Aves and Mammalia are warm-blooded; fishes, amphibians and reptiles are cold-blooded.
    • Mammals have hair and mammary glands and are mostly viviparous.
✎ Quick Check — 2 questions0 / 2
Q1.Which feature is NOT one of the three defining chordate characters?
Explanation: Jointed legs are an arthropod feature; the three chordate features are notochord, dorsal hollow nerve cord and gill slits.
Q2.Mammary glands and hair on the body are characteristic of:
Explanation: Hair and mammary glands are defining features of mammals.