This was fun, but there were a couple things I noticed:
1) The sun pieces you get from the "Vitality" upgrade don't seem to work toward the level goal (damaging the boss or charging the portal). If this is intentional, it should probably be included in the upgrade description.
2) The collision between you and your tail is a little weird. It looks like there is a small area near the cursor where there's no collision, but if this is intentional it would be nice if the rules were explained.
3) The amount of life and tail segments you lose when you get hit is hard to predict. I thought it was 1 life and as many tail segments as would be cut off if the enemy were a knife, but I'm not sure.
If the tutorial could be more clear about these things, this game would be perfect.
 "Shadow Snake 2"){kind=icon}