INHALING WITHOUT RIBS: THE PROBLEM OF SUCTION IN SOFT-BODIED INVERTEBRATES

¹ Department of Marine Science and Engineering, North Carolina State University, Raleigh, N. C. 27650

Most animals rely upon rigid skeletal structures to withstand the negative pressures required for inhalation. Some soft-bodied invertebrates (innkeeper worms, sea cucumbers) have pumping cloacae that are able to "inhale" without any hard supporting structures. Geometric analyses suggest that inhalation cannot be produced by relaxation of coelomic pressure and contraction of radial muscles connecting the body wall. Coelomic pressure must be maintained for body wall support and radial muscle antagonism. Mathematical models predict that maximum attainble suction in a hydrostatically supported sac-within-a-sac like the cloaca-body wall system will be proportional to coelomic pressure and exponentially related to the ratio of body wall diameter to cloacal diameter. A mechanical model gives results consistent with these predictions. "Cloacal" suctions are proportional to "coelomic" pressure, and maximum suction (20 x coelomic pressure) is produced when the cloaca is small relative to the body wall. In vivo recordings of pressure relationships during cloacal pumping in innkeeper worms (Urechis caupo) confirm that coelomic pressure is not relaxed to permit inhalation. No animals have been found which use a hydrostatically-supported system to produce extreme suction, with the possible exception of nematodes. Squids apparently utilize the potential of such a mechanism for producing large stroke volumes at low suctions with minimum structural complexity; the mantle is a sac-within-a-sac system which pumps large volumes for respiration and locomotion.