I had watched this the other day - really nice, it kept me interested. Congratulations on finishing it. I like the sky. The head is very well modeled. I like the voices. At 1;37 that was a good camera effect.
You have a clipping issue at 1:54-1:56 which is fixed by adjusting the near clipping plane or moving the camera back and adjusting the focal length. Is there any color to the ground? I feel like you could use just a little without losing the grey effect. I think maybe the subtitle font could be different; it looks too normal for the video.
When Head falls he should have to roll to get back up and he falls quite suddenly. It is also quite sudden when Worm opens his mouth gigantically. He maybe opens too much. You prolly know you have a face shading issue at 1;57-2;00 (on Head’s bald head) but that can be hard to fix. You might try adding a subdivision modifier and just turning it on for that part. Or move the light just a little bit.
It looks like bloody Head rolls in from afar, instead of emerging from Worm. Worm’s expression at 2;35 is priceless! And again at 3;17. I think the mouth glow is a bit too strong, I might try reducing the wattage about 50%. The ending noise with the black screen is very nicely done too.
Overall, the sound is impressive. I knew how important sound was when I made my first short. Done good, mate!
Oh yeah, Depth of Field. I think Worm might even be out of focus on his first crawl in. And overall I think the DOF is too strong, if it is needed at all. The reason for this is the background isn’t distracting at all and there aren’t a lot of objects to make the viewer confused where to focus their attention.