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Natural sciences

Life and way of living

Educator section


  • The crab
  • They have 10 legs.
  • The body consists of a cephalothorax and abdomen.
  • Skin covering is hard, like plastic.
  • The first pair of legs bears pincers.
  • The eyes are borne on stalks (two stalks with one eye each).
  • The mouth parts cut, are flat and are moved to propel water into the gill chambers.

Research assignment:

Provide the following guidelines:

Respiration involves effective gas exchange: oxygen and carbon dioxide.

The surface for gas exchange must

- Remain moist

- Be as large as possible

Now compare the adaptation of the four named animals in this regard, as well as the ways in which their adaptation helps (or doesn’t help) them to survive under different conditions.

Test your knowledge:

  • 10
  • gills, gill chambers, water
  • exoskeleton, calcium
  • cephalothorax and abdomen.

Leaner section


Activity 10: to study the crab and compare various invertebrates [lo 1.1, lo 1.2, lo 1.3, lo 2.3]

The crab

Crabs belong to the group of crustaceans (shell-fish) or Crustacea , which also includes lobsters, shrimps, prawns and mussels. Most members of the group are marine animals.

Can you still remember how the crustaceans differ from the insects and the arachnids?

They all have_____________________________________________________ legs.

Try to get hold of a crab and look at its external characteristics.

Can you distinguish between a head, a thorax and an abdomen? (Tip: the abdomen of a crab is folded underneath its body)


What does the skin-covering feel like? _____________________________________________________________________


What does the front pair of legs look like? _____________________________________________________________________



the eyes: _____________________________________________________________

the mouthparts: ________________________________________________________


The body of the crab consists of a cephalothorax and an abdomen. The abdomen is small and is folded underneath the cephalothorax. The following are found on the cephalothorax:

  • Five legs, of which the front pair has pincers;
  • Two eyes on stalks;
  • Two feelers; and
  • Mouthparts.

The skin covering (exoskeleton) is hard and like plastic. It consists of chitin that is strengthened with calcium.


Crabs move sideways and are very agile. Have you ever tried to catch one on the beach? It should not be very difficult for them to catch their prey!


Crabs are meat eaters (carnivores) and catch their prey with their pincers, with which they hold the food while eating it.


Crabs breathe with their gills and therefore are dependent on water. However, the gills are situated in gill-chambers that are filled with water all the time. Crabs therefore can also breathe on land.

Research assignment

Determine what is meant by the term respiration. Refer specifically to gas exchange and the gases involved. Then compare the earthworm, grasshopper, spider and crab with regard to their breathing. Make use of simple illustrations. Do the assignment on a loose sheet of paper that can be placed in your portfolio. Explain which of the animals will be able to survive under the greatest variety of conditions.

Maintaining a water balance

The exoskeleton of the crab prevents excessive water from moving into the body (when they are in water) or out of the body (when they are on land).


The crab lays eggs that hatch in the space between the abdomen and the cephalothorax. The young look just like the adults when they hatch.

The crab and other crustaceans in the ecosystem

Crabs and other crustaceans are meat eaters and form an important link in the food chain. They are adapted to live in water, but land crabs can survive out of the water for long periods.

Test your knowledge:

The crab

has _____________________________________________________________ legs;

breathes with __________________________ are found in _____________________

that are filled with ______________________________________________________

has an ______________________ that is strengthened by ______________________

that occurs in the chitin;

__________________has the following in common with spiders but not with insects:


Learning Outcome 1: The learner will be able to act confidently on curiosity about natural phenomena, and to investigate relationships and solve problems in scientific, technological and environmental contexts.

Assessment Standard 1.1: We know this when the learner plans investigations;

Assessment Standard 1.2: We know this when the learner conducts investigations and collects data;

Assessment Standard 1.3: We know this when the learner evaluates data and communicates findings.

Learning Outcome 2: The learner will know and be able to interpret and apply scientific, technological and environmental knowledge.

Assessment Standard 2.3: We know this when the learner interprets information.

Questions & Answers

what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Natural sciences grade 7. OpenStax CNX. Sep 16, 2009 Download for free at http://cnx.org/content/col11077/1.1
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