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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

how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
can nanotechnology change the direction of the face of the world
Prasenjit Reply
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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