2.2.1 skeletons

Skeletons

Skeletons in animals:

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Skeleton – interactive animation

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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