COELOMIC PRESSURES IN SIPUNCULUS NUDUS

¹ Marine Biological Laboratory, Woods Hole, Mass., and Station Biologique, Roscoff, France

The relationship between coelomic pressures and body movements has been investigated in Phascolosoma gouldi and, mainly, Sipunculus nudus.

During "defense movements, pressure in Phascolosoma is commonly 50 cm. of water, in Sipunculus 70 to 80 cm. During "defense immobility," the highest pressure recorded, in Phascolosoma, was 108 cm. There does not seem to be a relation between body size and maximum pressure.

Six phases of burrowing movements have been distinguished in Sipunculus and related to coelomic pressure. An important pressure rise coincides with a bending movement of the central part the body. This "hump" and the high pressures associated with it have functions which are described. Very low coelomic fluid pressures suffice to cause evension of the proboscis. Pressure is a necessary and sufficient condition for proboscis eversion in free water, while in sand the proboscis muscles seem to play an essential role during proboscis eversion in relation to the thixotropic property of stirred sand.

During most experiments, minimum pressures were very constant, while maximum pressures varied a great deal. During burrowing movements in free water, the highest pressure recorded in Phascolosoma was 25.0 cm., in Sipunculus 39.6 cm. of body fluid. During burrowing in sand, pressure of 96 cm. water was reached by Sipunculus.

The frequency of pressure cycles seems to be independent of the absolute pressure.

The coordination between the cycles of pressure and proboscis movements is variable. Therefore the retractor muscles and the muscles of the body wall are probably dependent on nervous centers which are not closely associated.

Pressure variations during burrowing have an influence on circulation of coelomic fluid.