Simple Animals – Sponges, Jellies, & Octopuses: Crash Course Biology #122

Hank introduces us to the “simplest” of the animals, complexity-wise: beginning with sponges (whose very inclusion in the list as “animals” has been called into question because they are so simple) and finishing with the most complex mollusks, octopuses, and squid. We differentiate them by the number of tissue layers they have, and by the complexity of those layers.