FUNCTIONAL-ORGANIZATION OF DERMAL CELOMIC CANALS IN SIPUNCULUS-NUDUS (SIPUNCULA) WITH A DISCUSSION OF RESPIRATORY DESIGNS IN SIPUNCULANS

Sipunculus nudus is a large sipunculan that burrows actively in porous marine sands. The body surface is folded into approximately 30 longit udinal ridges that extend along the length of the trunk and introvert. A dermal canal lies beneath each of the ridges and circulates coelomi c fluid and hemerythrocytes to within a few micrometers of the body su rface. Coelomic fluid enters and leaves the dermal canals through pore s leading from the trunk coelom and hows unidirectionally at approxima tely 0.7 mm/s from posterior to anterior. The flow is generated by cil ia on peritoneal cells lining the canals. The volume of the canals is approximately 8% of the total coelomic volume. We suggest that sipuncu lan species fall into three functional categories based on the body re gion or regions used for environmental gas exchange. ''Tentacle breath ers,'' such as species of Themiste, bore into rocks or micro-oxic sedi ments and extend their numerous tentacles into the water, Dissolved,ea ses are transported between the tentacular and trunk coelomic cavities via well-developed contractile vessels. ''Tentacle and introvert brea thers,'' such as species of Aspidosiphonidae and Phascolosomatidae, lo dge the trunk region of the body in the substratum and extend an elong ated, thin-walled introvert and tentacles into the water. Their contra ctile vessels are usually simple unbranched tubes. ''Integumentary bre athers,'' such as Sipunculus nudus and other species of Sipunculidae, have integumentary specializations for gas exchange across the entire body surface, but usually small tentacles and often weakly developed c ontractile vessels.