Practice question

Flatworm digestive tract remains incomplete, possessing single ventral or anterior mouth opening into blind gastrovascular cavity lacking anus, distinguishing from complete tube. Food enters same orifice, digestion proceeds extracellularly in lumen and intracellularly within gastrodermal cells, undigested residues egested through mouth. Cavity may be sac-like in Acoela, bifurcated in triclad planarians or extensively branched in polyclads increasing absorptive surface area. Parasitic cestodes secondarily lost gut entirely absorbing nutrients across tegument via facilitated transport. Evolution of complete through-gut with separate anal opening in nematodes and higher phyla improves directional processing, regional enzyme specialization and continuous feeding unavailable to Platyhelminthes.