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What are crustaceans?

There are over 50 000 species of crustaceans in the world. Most species are aquatic such as crabs and lobsters, however some are terrestrial such as slaters and pill bugs. They range in size from the 0.2mm microscopic water flea to the 4.3m leg span of the Japanese spider crab.

Soldier crabs like all crustaeans, belong to a larger groups of animals called arthropods. This means they have a segmented body covered in an exoskeleton and pairs of jointed limbs.

There are over a million known arthropod species. Arthropods include insects, arachnids, myriapods and crustaceans.

Case study: Pill bug


The pill bug, or rollie pollie, gets its name from being able to roll into a tight ball. It has a carapace that looks like armour which can vary in colour from white to dark grey. 


Pill bugs live in wet or damp environments. They are found under logs and rocks and amongst decaying vegetation.


Pill bugs eat algae, moss, bark, fungi as well as decaying plant and animal material.

Role in the Ecosystem

Pill bugs clean up the environment by helping to recycle and decompose old plant and animal matter. They are also food for centipedes, spiders, ants, birds and amphibians. 

Pill bug anatomy


The head and first segment of the thorax are joined together. This is called the cephalothrax.


The thorax includes the region between the 2nd and 7th segment. This region is called the pereon.

The hard exoskeleton has 7 armoured plates which protect the body. They are made of chitin.



The abdomen is made up of 5 condensed segments called the pleon.

Find out more

The invertebrate explorer digital book explores the incredible world of Australian invertebrates.

Students can use the book to investigate classification, features, adaptations and habitats of a variety of Australian invertebrates through narrated videos, stunning images, interactive activities and detailed text.

This book was designed by teachers to support the NSW Science and Technology K-6 syllabus and NSW English K-6 syllabus.

Content supports living world, Australian animals and class studies on invertebrates.

Download for free from Apple Books.

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