In this episode, the gang decides to go back to some old papers they enjoy to discuss the concept of homology. What do we really mean when we say certain characteristics are shared due to evolutionary history? Also, for a brief minute in the second half, James was spontaneously possessed by demonic spirits that made him spout nonsense he doesn't actually believe. Unrelated to this, he also had a splitting migraine. 

Up-goer Five (Amanda in a fever-based fugue state edition): 

Today our friends talk about how things are the same because animals are brother and sister. This means that the brother and sister animals have parts that are the same because they have the same mother and father animals. But the way that brother and sister animals have the same parts can be because of different ways. People do not understand really what it means when we say that these brother and sister animals have the same parts. So our friends try to explain how these parts came to be and why.

References:

Van Valen, Leigh M. "Homology and causes." Journal of Morphology 173.3 (1982): 305-312.

Wiley, E. O. "Homology, identity and transformation." Mesozoic fishes 4 (2008): 9-21.