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
This course has nine lessons.
- Scope and Nature of Invertebrate Animals
- 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
- Microscopic Animals
- Protozoa or Animalia
- Phylum Nematoda
- Phylum Tardigrada
- Adaptability and Survival
- Phylum Kinorhycha
- Phylum Loricifera
- Phylum Placozoa
- Worms & Worm Like Animals
- True worms vs worm-like organisms
- Worm evolution
- Bilateral symmetry
- 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
- Sponges, Corals, Anemones, Jellyfish
- Phylum Cnidaria
- Cnidaria and Humans
- Phylum Ctenophora
- Phylum Porifera: Location, Internal & External Structures, Reproduction, Toxicity
- Classes within Porifera
- Finding food
- 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
- Arthropods 1
- Classification into Arachnida, Crustacea, Myriapoda and Insecta (insects)
- Characteristic body parts
- 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)
- Arthropods 2
- Crustacea (Crustaceans)
- Class Malacostraca: Crayfish, Crabs, Shrimp etc
- Branchiopoda: Fairy shrimp, Water fleas
- Sessile Crustaceans
- Sub Phylum Uniramia: millipedes, centipedes and insects
- Insects 1
- Origin of insects: winged vs non-winged
- Class Entogantha: Collembola, Diplura, Protura
- Class Insecta
- Insect features
- Insect classification into 29 orders
- Specialised organs
- Senses: vision, comminication
- Odonata: Dragonflies and Damselflies
- Mantodea: Mantises
- Orthoptera: Grasshoppers, Crickets, Katydids
- 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.
for protecting inner parts that covers the head, head and thorax, or the entire
Dioecious: separate sexes i.e. individuals possess either male or female reproductive
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
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 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
- 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
- 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
- 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
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).
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.
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.
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.
Most arthropods have a pair of compound eyes and one or more
simple eyes (ocelli). In some arthropods eyes are absent or reduced.
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.
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
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
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
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.