What is the structure called, found in Agnathans, which will become the endoskeleton in more evolutionary advanced organisms?

Hydra do not have an endoskeleton. They are multicellular

organisms that can be found in unpolluted fresh-water ponds,

streams, and lakes.

the answer is lactic acid from anaerobic respiration. the acid in lactic acid is what creates sore and burning muscles. if you think about how acid burns that will give you a clue for questions like this.