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Skeletons
Skeletons in animals:
Skeleton – interactive animation
Overview
- 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
Read also:
OpenStax, Siyavula: life sciences grade 10. OpenStax CNX. Apr 11, 2012 Download for free at http://cnx.org/content/col11410/1.3
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