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In short, animal bodies can be constructed either as a chain of repeated basic structures called segments or otherwise. We could think for example in an earthwo
by pvaldes 3y ago
In short, animal bodies can be constructed either as a chain of repeated basic structures called segments or otherwise. We could think for example in an earthworm and a jellyfish.
All segmented animals can be represented as a list of ordered segments. Different lists can have different lengths, but the number of elements inside each type of animal is very stable. We are segmented animals also.
An hypothetical animal with five segments in its body plan, would be represented as:
'(1 2 3 4 5)
Each element in (cdr '(1 2 3 4 5)) has the ability to grow a couple of structures called appendix. The first segment will bear another special type of sensors designed to detect light, the eyes. Animals use chemical gradients to modulate the appendix separately and turn them into everything that will need, in the right place.
The concept of chemical gradients is not difficult to understand. Drop some chemical in the first segment and let it to diffuse towards the end of the chain. The first part will receive much more chemicals than the tail, activating different genes. Our embryo now looks like this:
'(head torax abdomen)
Rinse and repeat for each element, nesting lists. This very smart process is all that we need to make an animal while guaranteeing that our embryo will never develop a couple of ears in the legs [1].
[1](... Unless is an arthropod, because each type of animals have their own ways to solve the problems)
- pvaldes 3y agoArthropods are a very species rich and very diverse group of animals, but using this system we can safely classify them into several big categories by the type, class and position of its appendix in the list. In the real life this translates to: '( antenna leg leg leg) This is an hexapod, for example an ant, but: '( fake-antenna leg leg leg) This is Myrmarachne a spider mimicking an ant. It moves its first pair of legs to simulate an antenna and trick its preys, but we know that the structure is different. If you are an ant, you would be dead by now. '( antenna antenna antenna leg leg) This animal does not exist. Not in this planet. Is an alien. If we have a partial fossil with one leg and a couple of wings, we can say that this was a modern insect. Wings developed only in the modern insects. The appendix and its location are so different among the main groups of arthropods and so fixed inside the groups that we can safely say that trilobytes didn't fall in any known category of alive arthropod. Those were not crustaceans and all went extinct. We also know that spiders are not just crustaceans that lose a couple of legs and a couple of antennas. Its legs are placed in different segments, and the auxiliar structures in its head are totally different.