Endoskeleton vs chitinous exoskeleton

[fran008]

In an attemp to try an explan how the adaptation of an endoskeleton have increased the evolotionary success of the organisms that posses them, my text book gave the following reasons :

Although stiffness provides support for muscles, exoskeleton is not as strong as an endoskeleton

Endoskeleton does not limit space available for internal organs and can support greater weight.

The problem is that I do not uderstand why this is the case.

Can anyone help?

[mith]

for which point? the first or the second?

[canalon]

1- exoskeleton is made of protein, endoskeleton of minerals, the latter is much stronger than the other.
2- the problem is that you cannot increase size of exoskeleton to just fit more inside, because this needs an increase in thickness to compensate for accrued weight. And to be strong enough the increase of thickness soon become more bulky that whatever space you create in it, and the animal is not viable anymore.

[fran008]

"because this needs an increase in thickness to compensate for accrued weight." why is this so if I may ask again?

[canalon]

Sorry english is not my native language. What I was trying to say is that if your animal is growing, and hence gain weight, to keep the same qualities of stiffness/strength the exoskeleton needs to get thicker and thicker. So thick that in fact most of the body mass for human sized bug would be exoskeleton. And this would not be viable (think how insect do respire through their exoskeleton)

[fran008]

Yeah yeah tnx loads. And organisms possesing exoskeletons have also another disadvantage...that of heaving to shed iot to grow and thus be very vulnerable at this stage. Can the organisms move without exoskeleton when they are moulting?

[Carolynparsons]

Crab moult but still move around, that's one of the problems with the Newfoundland and Labrador crab fishery (refering to the queen crab or snow crab). Soft shelled crab being caught in the crab pots. It's highly unlikely that the crab goes into moulting in the pot, therefore it must move to get in there while moulting.

Also sea organisms with exoskeletons can grow to be larger then land organisms because their weight is supported by the buoyancy in the water.
I'm aftering catching lobster that were close to 2 feet in length, with claws bigger than my hand.