The word support means to hold upright or to give strength
Aquatic organisms get support from water. Plants and animals living on land
require stronger support systems as air gives their bodies little supportagainst the force of gravity.
Animals have skeletons that support their bodies and enable movement to occur.
In animals movement is brought about by the contraction or relaxation of
muscles. In order to bring about movement muscles need a medium (fluid) orsolid structure against which the force of contraction can be applied. A
skeletal system provides the resistance needed to cause movement.
Animals are able to move from one point to another to look for food, shelter and
mates.
The simplest invertebrates have specialised cells and tissues to assist them to
move to and from stimuli.
Skeletons also have a protective function in that they cushion vital organs
thereby preventing or limiting damage.
Hydrostatic skeleton
It consists of a fluid-filled cavity enclosed by the muscles of the body wall
The fluid presses against the muscles, that contract against the pressure of the
fluid
So, a combination of the pressure of the fluid and the contracting muscles, can
alter the shape of the animal and allows for movement
If the body is segmented the pressure of the fluid is localised in a few
segments at a time.
Occurs in flatworms, round worms, earthworms starfish, slugs etc.
Note that starfish and other Echinoderms have an outer skeleton of calcareous
ossicles or spicules for protection . This outer skeleton encloses a watervascular system with tube feet that are moved by fluid pressure changes i.e
serves as a hydrostatic skeleton which controls movement.
Advantages:
allow the animal to move in a more flexible manner
fluid cavity stimulates circulation in the animal
allows for change of shape e.g. earthworm
Disadvantages:
dehydration will affect the skeleton directly and the ability of the animal to
move because of the loss of shape
does NOT provide protection for the internal organs
does not allow for rapid movement.
Exoskeleton
This forms the outer covering of the animal
The skeleton is made of a substance called chitin, secreted by the epidermis
It is confined to insects, spiders, scorpions, crabs etc all of which belong to
the Arthropod group (jointed legged animals
The abdomen is soft and attached to the thorax
The exoskeleton acts as a hard outer covering to animals and is made up of a
series of plates or tubes.
Muscles are attached to the inside of the exoskeleton which provides the
resistance needed for muscle action.
Advantages:
forms the point of attachment of internal muscles needed for locomotion and
flight
supports and protects the delicate inner parts of the animal
prevents desiccation (drying out) on land
has a low density and is therefore lightweight, to allow for flight
mouthparts can be modified for biting, sucking, piercing
grasping etc.
Disadvantages:
final body size is limited because as the body size increases, the surface area
to volume ratio decreases. The larger the animal, the heavier the exoskeleton,making movement more difficult.
growth is restricted, so periodic moulting is required if the animal is to grow
very vulnerable when it is in the moulting process, as it cannot move until the
exoskeleton is dry and has hardened
