RESPIRATION 



139 



evaginations of the body wall (papulae), each of which contains a central 

 cavity communicating with the perivisceral coelom. External cilia maintain 

 a current of sea water over the surface of the animal, and gases are ex- 

 changed with the gently circulating coelomic fluid across the walls of the 



Fig. 4.1. Section through the Dorsal Body Wall of the 



Lug worm Arenicola marina, Showing Branchial Tufts 



(From Wells, 1944) 



Tentacles 



Bra.nchia.1 

 crown 



Fig. 4.2. Gills of Polychaetes 



{Left) anterior region of the terebellid Lanice conchilega, showing feeding tentacles 

 and gills. (Right) sabellid worm Myxicola infundibulum. Branchial crown protruding 

 from orifice of gelatinous tube. (After Mcintosh, 1922.) 



gills and tube feet (20). Experimentally it has been shown that the ambu- 

 lacral system is important in oxygen uptake, which decreases if one or 

 more ambulacral grooves are sealed. About half the respiratory exchange 

 takes place through the tube feet in the normal animal, the remainder 

 occurring through the dermal branchiae (1 14). In echinoids the peristomial 

 region is ringed with similar branched gills, which communicate internally 

 with the lantern coelom. 



