Practice question

Echinoderms are predominantly ammonotelic, reflecting marine osmoconforming habitat allowing direct diffusion of toxic ammonia. Protein catabolism generates ammonia which is highly soluble and permeates across thin respiratory surfaces such as papulae, tube feet and cloacal lining into surrounding seawater without requiring enzymatic conversion. This conserves ATP otherwise spent converting ammonia to urea in mammals or uric acid in birds and insects, which are adaptations for water conservation on land. Continuous water flow removes ammonia, preventing accumulation, aligning with limited specialized excretory organs and simple diffusion-based nitrogen elimination in echinoderms.