Ideal animal build?

There is no universal ideal. The number of limbs is an adaptation to a particular environment. It is also a product of ancestry. All tetrapods (amphibians, reptiles, birds and mammals) have 4 limbs or less because their common ancestor had 4 limbs. To increase the number of limbs, a species would have to evolve a new body segment, because the body plan codes for 2 limbs per body segment in vertebrates. A new body segment is possible, but that is a major evolutionary change in the body plan and it has not happened in any tetrapod species in history. otoh, reduction in limbs is more common. Snakes lost all 4 limbs, but the oldest snake fossils still showed the presence of 2 rear limbs. so, even a reduction in limbs may take multiple stages. Whales and dolphins have also lost their rear limbs, as did manatees and dugongs, as their hindlimbs are useless, since all of these aquatic mammals swim with their take flukes, and the rear limbs are not only not necessary but they are also harmful since they make the animal less streamlined, slowing down the animal and increasing the amount of energy needed by the animal to swim faster.

The common ancestor of insects have six legs, 2 per body segment. To increase the number of legs, they too have to increase the number of body segments. Once again that has not happened in any species in the history of their evolution. Once again, some insects have evolved fewer legs, namely 4. So, both the examples of insects and tetrapods show that not only is an increase in legs not likely, a reduction in limbs, as adaptation to the environment, is actually more common. If so, then why do millipedes have so many legs and body segments? Millipedes and centipedes have so many legs because they have lots of body segments, and they also retain the primitive body plan of arthropods in haing 2 pairs of limbs (=4) per body segment. Millipedes and centipedes are also among the most primitive arthropods. They have changed little since they first evolved. That in turn means that the common ancestor of all arthropods was more like milipedes than insects or spiders. Since then, the different lineages have tended to evolve fewer body segments and fewer limbs. Crabs and shrimps are also arthropods. Although they have more limbs than insects and spiders, they neverteless have far fewer limbs than millipedes and centipedes.

Therefore it would appear that the body plans of most living animals started with more body segments and more limbs. From that point onward, the many evolutionary lineages tended to lose body segments and limbs when they adapt to the environment. Far more common is the retention of the number of body segments in limbs. Only a tiny percentage of insects have fewer than 6 limbs. Only a tiny percentage of mammals have 2 or fewer limbs. Snakes are a successful group with thousands of living species, but they had a single common ancestor, which was a lizard. Therefore among the tens of thousands of lizard species that have ever lived, only a handful of species evolved to be limbless, since there are some limbless lizards too. The reason for so relatively few changes in number of limbs is because organisms usually make do with what they have inherited, and changes are only made if there are good reasons, and only some changes are likely.

Problem 1:

Ideal for WHAT??