Gills in Protochordates

• A large and sieve-like pharynx in majority of these animals performs dual function of respiration and trapping food particles which are brought in through the current of water.
• The primitive pterobranch hemichordates (Cephalodiscus and Rhabdopleura) have either no gill slits or have very few and sport tentaculated arms, which other than food gathering, also function as efficient respiratory organs.
• The free-living urochordates, such as Salpa and Doliolum do not possess many stigmata or gill slits as their entire body is permeable to oxygen but in the sedentary ascidians pharynx is prominently enlarged and perforated with no less than 200,000 stigmata for filter-feeding.
• Cephalochordates use pharynx for both filter-feeding and respiration and hence carry 150-200 pairs of gill slits.