Practice question

Pogonophorans encompassing small perviate frenulates and large obturate vestimentiferans within Siboglinidae exhibit complete secondary loss of digestive system in adult stage after metamorphosis lacking mouth gut and anus entirely. Evolutionary rationale lies in obligate mutualism with chemosynthetic sulfur-oxidizing bacteria housed in trophosome tissue and capacity of epidermis to absorb dissolved organics directly from seawater. Maintaining energetically expensive gut useless when symbionts supply all requirements. Small frenulates inhabit reducing sediments absorbing sulfide across tube while large vestimentiferans rely exclusively on bacterial chemosynthesis. Therefore gutlessness reflects highly derived adaptation to chemosynthetic habitats not primitive absence of gut structure in ancestral annelids.