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 molluscs, octopuses and squid. We differentiate them by the number of tissue layers they have, and by the complexity of those layers. Table of Contents: 1) Porifera 1:33 2) Cnidaria 2:36 a) Diploblasts 2:48 3) Platyhelminthes 3:33 a) Triploblasts 3:56 b) Coelom 4:36 4) Biolography 5:36 5) Nematoda 7:26 6) Rotifera 7:57 7) Molusca 8:33 References: http://www.ucmp.berkeley.edu/cnidaria/cnidaria.html http://www.slate.com/articles/health_and_science/science/2008/06/how_smart_is_the_octopus.html http://en.wikipedia.org/wiki/File:Dicrocoelium_LifeCycle.gif http://entnemdept.ufl.edu/creatures/nematode/soil_nematode.htm http://soils.usda.gov/sqi/concepts/soil_biology/nematodes.html http://www.eurekalert.org/pub_releases/2010-08/uoc--goa080410.php http://www.ucmp.berkeley.edu/cambrian/cambrian.php http://www.insidescience.org/news-service/1-2615 http://www.wormbook.org/chapters/www_genomevol/genomevol.pdf http://www.cartage.org.lb/en/themes/sciences/zoology/animalclassification/Polygenetic/phylogenetictree/phylogenetictree.htm http://www.esu.edu/~milewski/intro_biol_two/lab_9_porifera_cnidaria/Cnidaria.html http://comenius.susqu.edu/biol/202/animals/protostomes/default.htm Crash Course is on Patreon! You can support us directly by signing up at http://www.patreon.com/crashcourse Want to find Crash Course elsewhere on the internet? Facebook - http://www.facebook.com/YouTubeCrashCourse Twitter - http://www.twitter.com/TheCrashCourse Instagram - https://www.instagram.com/thecrashcourse/ CC Kids: null