Our teachers specifically told us these gill slits were used for breathing fluid...along with Haeckel’s Embryo Chart in our text books, which was declared a fraud nearly 100 years earlier.
I must have been sick that day in biology class, because I never remember hearing about this stuff until years & years later.
To this day, I've seen only evidence that Haeckel was somewhat mistaken -- honest but mistaken -- not that he was deliberately perpetrating "fraud".