Nice prototype, sadly I hit escape just after beating "Spore Spawn", so I couldn't see what happened after defeating it :/

The design of the backtracking, while a little linear at the start is quite open overall, that's a plus, but sometimes the rooms feel like they were stitched together in an afternoon, (like the inconsistent block thickness and platform length), it probably would have benefited if everything was aligned on a grid.

While not the main focus of your project, the character felt a little clunky to control.
It's a shame because I saw good design elements of platforming, such as putting lower platforms under the gaps of higher platforms,
but the way the character sometimes bounces on the floor after a hit or keeps momentum after missing a jump makes it hard for the good design of the platforming to actually work.

Bombs take a little long to explode, and it's not like there was anywhere to bomb jump really... A glitch that happened to me a few times was that I couldn't unmorph when I was inside of a destroyed bombable block (see picture #2 of the post, the block on the right).

The boss was unnecessarily hard, but most of it was at least communicated well for a prototype, like the acid rain filling the room.
Although I would've liked the charged beam to break completely an attack instead of spliting it in 2 like a normal shot, but whatever.

Overall, nice experience that accomplishes more than what it promises to do.