flew through the space ameba in The Immunity Syndrome
, and Spock described it as...
SPOCK: "Readings coming in now, Captain. Length, approximately eleven thousand miles. Width varying from two thousand to three thousand miles. Outer layer studded with space debris and waste. Interior consists of protoplasm, varying from a firmer gelatinous layer to a semi-fluid central mass
. Condition, living."
So in TOS, the Enterprise
flew through a mass of fluidic-gelatinous material.