Animate transparency of entire layer Blender noob here. I'm learning my way around but could use some assistance. I'm working on a project having to do with human anatomy. I have a 3D model complete with skin, skeleton, muscles, organs and so on. Each of those are on a separate layer. I am trying to animate the transparency of the muscles however that layer is made up of several meshes/textures and I can't figure out how to simply fade them on screen. I have already done something similar with the skin layer but that is all one mesh. What's the easiest way to do this? Just so it's clear, I have my transparency set to about 20% on the skin to show the organs. See below 
I then want to fade the muscle group on so it looks like this. 

Is there a way to add keyframes to entire layer? I'd appreciate any help. Thanks!
I am not animation expert but I think what you want to do is... make your separate completed renders for organs, muscles, and then skin. then, in the compositor, create your fade-in animation from one completed render image to the next by using the alpha-over node. This example probably explains it better than I can - https://docs.blender.org/manual/en/dev/compositing/types/color/alpha_over.html