Fish subclade Updated 2025-07-16
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.
Kingdom (biology) Updated 2025-07-16
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.
Reproductive cue Updated 2025-07-16
A signal that indicates that an animal is ready for mating.
The simplest multicellular species Updated 2025-07-16
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.
Video 1.
Nicole King (UC Berkeley, HHMI) 1: The origin of animal multicellularity by iBiology (2015)
Source.
Vertebrate Updated 2025-07-16
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.