Practice question

Brachiopods lack specialized lamellar gills comparable to molluscan ctenidia; instead respiratory exchange occurs across extensive surface provided by lophophore tentacles and inner mantle epithelium lining internal shell cavity, both highly vascularized with ciliated epithelium and blood sinuses facilitating oxygen diffusion from seawater driven through cavity by ciliary beating. Mantle canals and lophophore sinuses serve exchange sites blood containing hemerythrin or hemoglobin. Diffuse mode suffices for low metabolic sessile lifestyle in well-oxygenated marine waters supporting ancient benthic habit without evolution of complex discrete gill organs or pulmonary surfaces, though efficient enough for ecological niche and persistence since Cambrian period.