Question

Why do the cartilageous fishes cannot excrete ammonia,while the bony fishes excrete ammonia?

Answer 1

cartilagenous fishes are ureolitic fishes
they excrete the waste in the form of urea as their undigested food mostly forms urea in their digestive system
and much of the urea is stored in the blood of cartilagenous fishes,hence they are even not edible,but bony fishes excrete ammonia,as they form less amount of urea from undigested food,it's natural..