How do gametes get into coelom, and, later out of it? Excretion can be under quite heavy pressure, see https://www.youtube.com/watch?v=1OsRbuEINXI
As question has not received answer, will answer to best of my ability. Eggs are plopped off from ovary, into coelom. Duct out of coelom is the oviduct that opens into it. The fish must apply pressure to the whole of the coelom, I guess, to push it out. That part is a bit of a mystery to me, an expert answer would be better.