Ciro Santilli @cirosantilli 35

Formal name: "animalia".

List of animal class: en.wikipedia.org/wiki/List_of_animal_classes

Once Ciro joked in a twenty questions-like game that humans are animals.

But counting humans a fish would have been a stroke of genius.

There's six to eight in different systems of the end of the 20th century:This mess is because people don't realize that clades are all that really matter.

A signal that indicates that an animal is ready for mating.

One of the simplest known seems to be: en.wikipedia.org/wiki/Trichoplax

www.u-tokyo.ac.jp/focus/en/articles/a_00220.html "The simplest multicellular organism unveiled" from 2013 mentions Tetrabaena socialis.

Then of course: Caenorhabditis elegans is a relatively simple and widely studied model organism.

The big breakthrough of the vertebrates appears to be the ability to swim around in a straight line and eat smaller species that are floating about.

Bones appear to help that a lot!

It is likely the most efficient design to travel long distances. Be thin and wiggle your tail around.

Perhaps smaller animals can skip the bone thing. Maybe a notable example are the lancelets, which look a bit like small fish. But they only go up to 8 cm.