1.9 General science

Natural sciences

Life and way of living

Educator section

Memorandum

• 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

Content

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? _____________________________________________________________________

_____________________________________________________________________

Describe

the eyes: _____________________________________________________________

the mouthparts: ________________________________________________________

Build

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.

Movement

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!

Feeding

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

Respiration

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).

Reproduction

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:

Assessment

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.