Invertebrate Zoology BEN218

Discover Invertebrates, the Most Numerous Group of Animals on Earth

Invertebrates — spiders, snails, insects, octopodes and more — are the largest group of animals on the planet. Divided into 8 distinct phyla, this group of animals has significant impacts on agriculture, human health, and even housing.

Invertebrates give us a window into the development of life, from the simplest life forms (e.g. tardigrades, or water bears) to some of the most beautiful (e.g. Odonata, the dragon- and damselflies). Develop an appreciation for these wonders, large and small.

Course Duration: 100 hours

Course Content

This course has nine lessons.

  1. Scope and Nature of Invertebrate Animals
    • Introduction
    • Significance to humans
    • Comparative studies: invertebrate animals
    • Important terminology
    • Overview of Invertebrate Phyla
    • Microscopic phyla" Tardigrada, Kinorhyncha, Loricifera, Placozoa
    • Worms: Acanthocephala, Annelida, Hemichordata, etc.
    • Corals and relatives: Cnidaria, Ctenophora, Ectoprocta, Porifera
    • Echinoderms and Molluscs: Echinodermata, Mollusca, Brachiopoda
    • Complex Invertebrates: Arthropoda
  2. Microscopic Animals
    • Protozoa or Animalia
    • Phylum Nematoda
    • Mites
    • Phylum Tardigrada
    • Adaptability and Survival
    • Anhydrobiosis
    • Cysts
    • Phylum Kinorhycha
    • Phylum Loricifera
    • Phylum Placozoa
  3. Worms & Worm Like Animals
    • True worms vs worm-like organisms
    • Worm evolution
    • Bilateral symmetry
    • Cephalisation
    • Body organisation
    • Characteristics and systems showing complexity
    • Phylum Platyhelminthes (Flatworms)
    • Free living flatworms
    • Parasitic flatworms
    • Significance to Humans: Liver fluke, blood flukes, tapeworms
    • Beef tapeworm
    • Phylum Nematoda (Roundworms)
    • Phylum Annelida (Segmented Worms)
    • Other Worm Like Animals: Acorn worms, ribbon worms, Spiny headed worms, etc.
    • Coelomate Worms
  4. Sponges, Corals, Anemones, Jellyfish
    • Introduction
    • Phylum Cnidaria
    • Hydrozoa
    • Scyphozoa
    • Cubozoa
    • Anthozoa
    • Cnidaria and Humans
    • Phylum Ctenophora
    • Phylum Porifera: Location, Internal & External Structures, Reproduction, Toxicity
    • Classes within Porifera
    • Finding food
  5. Molluscs and Echinoderms
    • Phylum Echinodermata
    • Crinoidea: Sea Lilies and Feather Stars
    • Ophiuroidea: Brittle stars, Basket Stars
    • Asteroidea: Sea stars or Starfish
    • Case Study: Crown of Thorns Starfish
    • Echinoidea: Sea urchins, Heart urchins, Sea dollars
    • Chass Holothuroidea: Sea Cucumbers
    • Phylum Mollusca: general characteristics and types
  6. Arthropods 1
    • Classification into Arachnida, Crustacea, Myriapoda and Insecta (insects)
    • Origin
    • Terminology
    • Characteristic body parts
    • Ecdysis
    • Digestion, Respiration, reproduction and other systems
    • Phylum Arthropoda
    • Chelicerata (Chelicerates)
    • Arachnida (Scorpions, Spiders, Mites and Ticks)
    • Scorpiones (Scorpions)
    • Araneae (Spiders)
    • Acari (Mites and Ticks)
    • Opiliones (Daddy Long-Legs)
    • Merostomata (Horseshoe crabs)
    • Pycnogonida (Sea spiders)
  7. Arthropods 2
    • Terminology
    • Crustacea (Crustaceans)
    • Class Malacostraca: Crayfish, Crabs, Shrimp etc
    • Branchiopoda: Fairy shrimp, Water fleas
    • Cephalocardia
    • Remipedia
    • Maxilopoda
    • Sessile Crustaceans
    • Sub Phylum Uniramia: millipedes, centipedes and insects
  8. Insects 1
    • Origin of insects: winged vs non-winged
    • Class Entogantha: Collembola, Diplura, Protura
    • Class Insecta
    • Insect features
    • Mouthparts
    • Insect classification into 29 orders
    • Specialised organs
    • Reproduction
    • Lifecycle
    • Senses: vision, comminication
    • Odonata: Dragonflies and Damselflies
    • Mantodea: Mantises
    • Orthoptera: Grasshoppers, Crickets, Katydids
  9. Insects 2
    • Significance to man
    • Clean air and water
    • Pollination by insects
    • Edible insects
    • Case Study: Grasshoppers save lives
    • Order Diptera: Mosquitos and Flies
    • Order Hymentoptera: Bees, wasps, ants, sawflies
    • Order Coleoptera: Beetles, weevils

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Why Do Invertebrates Matter?

Invertebrate animals are very important animals to man.

  • Bees pollinate our food crops and give us honey.
  • Worms improve our soils. 70% of the world human population eat insects.
  • Without invertebrates, many of the larger animals and plants could not survive.

We need to understand and manage invertebrates well in order to have a sustainable global environment.

Where Do Arthropods Come From?

It is thought that arthropods originated from primitive segmented worms or from a common ancestor as they share a number of distinguishing characteristics with annelid worms. Like annelids, arthropods have segmented bodies. While the body segments of annelid worms are structurally and functionally similar, the body segments of arthropods have fused throughout evolution, and produced distinct and specialised segments, with each segment bearing a pair of specialised appendages.

Important Definitions

Carapace: shield for protecting inner parts that covers the head, head and thorax, or the entire body.

Dioecious: separate sexes i.e. individuals possess either male or female reproductive organs.

Mandibles: Mouth-parts most commonly used for seizing and cutting food.

Maxillae: Mouth-parts used in eating. The maxillae are paired limb-like structures located immediately behind the mandibles, which articulate with the head capsule.

Characteristics of Arthropods

There are several distinguishing features that most arthropods share:

  • Body Form
  • Some body segments are fused into three specialised sections (tagmata), the head, thorax and abdomen
  • The arthropod body is bilaterally symmetrical


The arthropod body is completely enclosed by a tough outer covering called the exoskeleton. The exoskeleton is mainly composed of a polysaccharide called chitin within a protein matrix, as well as other proteins, lipids and fats. The cuticle contains the predominant materials of the exoskeleton.

The exoskeleton functions to support and protect the arthropod body. It provides anchor points for muscles and it is relatively impermeable to water. Due to the physical and chemical properties of the exoskeleton, arthropods are unable to grow continuously. Arthropods undergo periodic growth where growth occurs during specific phases and then the exoskeleton is shed.


Arthropod growth occurs through ecdysis (moulting of the exoskeleton). As the animal grows, the exoskeleton increases in thickness until it can no longer stretch or change shape, forming a barrier to further growth. The exoskeleton is then shed, a process called ecdysis (moulting).

Ecdysis is controlled by the hormone, ecdysone. The process begins with the separation of the cuticle from the epidermis. Mineral salts are withdrawn from the old cuticle and the epidermis secretes a new cuticle, and the old cuticle is shed. The new cuticle is very soft and pliable and the arthropod will generally intake air or water into its body to inflate and increase the size of the cuticle. The final step involved is for the new cuticle to harden – this occurs through dehydration.

The new arthropod cuticle is usually very pale in colour. As the exoskeleton hardens, it becomes darker or gains colour.

The process of ecdysis is energetically costly for arthropods. It is also a risky process, with arthropods being vulnerable to predators during and shortly after moulting. There are, however, a number of advantages to ecdysis:

  • Ecdysis is the basis of metamorphosis life cycles, where the larval and adult animals are different and have different ecological roles, e.g. maggots compared with flies.
  • Ecdysis promotes body repair. If an arthropod is injured in an early phase of life, for example, a crab losing a claw, the injury may be repaired after a couple of moulting cycles.
  • Ecdysis allows the periodical replacement of fragile body parts, for example, the urticating hairs of tarantulas or caterpillars.


Appendages are paired, jointed and segmental.

Paired appendages were present on all body segment in ancestral arthropods. The appendages of living arthropods have evolved to be extensively modified or even lost in some species.


The locomotion of arthropods is predominantly controlled by a complex muscular system (smooth and striated muscle) attached to the appendages.

Body Cavity

Arthropods are eucoelomate, meaning that they have a coelom (fluid-filled body cavity), but it is reduced. In the case of arthropods, the coelom is typically found in sections of the excretory and reproductive systems. The body cavity of arthropods is primarily a haemocoel (space containing haemolymph i.e. circulatory fluid, predominantly blood).

Circulatory system

Arthropods have an open circulatory system comprising a heart, arteries and the haemocoel. The heart forces haemolymph through the arteries to the organs and the "blood" returns to the heart through valved pores. Some groups of arthropods possess additional tubes for the passage of oxygen to the organs.

Nervous System

The arthropod nervous system is similar to that of annelid worms comprising a dorsal brain (cerebral ganglion) and ventral nerve cords (longitudinal chains of segmented ganglia) connected by a nerve ring enclosing the pharynx. Lateral nerves extend from the nerve cords into the different segments of the body.

Sense Organs

The sense organs of arthropods (eyes for sight; olfactory receptors for smell; antennae for touch) are well-developed and usually positioned on the anterior of the organism.

Compound eyes

Most arthropods have a pair of compound eyes and one or more simple eyes (ocelli). In some arthropods eyes are absent or reduced.

Respiratory System

Arthropods do not have a predominant respiratory organ like most other animals. Instead, the passage of gases occurs through the body surface of the animal, usually with the aid of additional structures.

Aquatic arthropods usually have gills that are thin, feathery outgrowths of the skin providing a very large surface area. Being an extension of the skin, the gills are covered by the exoskeleton; the exoskeleton is, therefore, reduced in thickness in this locality to allow effective gas exchange.

Terrestrial arthropods typically have internal structures specialised for gas exchange, for example trachea (branched air ducts linking pores in the cuticle to the inside of the organism) or book lungs.

Digestive System

In arthropods, the gut is complete. The structure of the arthropod digestive system is variable, depending on what and how the animal eats. Generally, they have a foregut and hindgut with a chitinous lining (the same as the exoskeleton material). This lining moults in conjunction with ecdysis of the exoskeleton. They also have a midgut where the chitinous layer is absent. It is here where the majority of enzyme production and absorption takes place.

Excretory System

Arthropods may have paired excretory organs that eliminate waste at the bases of particular appendages (crustaceans and some arachnids). Other arthropods (myriapods, insects and other arachnids e.g. spiders and mites possess excretory organs called Malpighian tubules that feed into the intestine, meaning that both excretory and digestive wastes are eliminated via the anus.


Most arthropods are dioecious (i.e. the sexes are separate, so individuals possess either male or female reproductive organs).  The reproductive organs (gonads) are paired (ovaries or testes). Internal fertilisation is predominantly internal and most lay eggs.

In some groups, the young that hatch resemble the adult form; in other groups, particularly insects, the young hatch as larvae and further development takes place with metamorphosis.


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