. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. INVERTEBRATE SUCTION 191 prevent collapse of the body wall, and so forth. The cloacal breathers appear to be like the man attempting to lift himself by his bootstraps. The solution to this apparent paradox lies in the geometry of the animals. The total force preventing collapse of the body wall around the cloaca is equal to the difference between coelomic and ambient pressure times the area of the body wall available for attachment of radial muscles, whereas the total force hindering ex- pansion of the cloaca is the differen
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. INVERTEBRATE SUCTION 191 prevent collapse of the body wall, and so forth. The cloacal breathers appear to be like the man attempting to lift himself by his bootstraps. The solution to this apparent paradox lies in the geometry of the animals. The total force preventing collapse of the body wall around the cloaca is equal to the difference between coelomic and ambient pressure times the area of the body wall available for attachment of radial muscles, whereas the total force hindering ex- pansion of the cloaca is the difference between cloacal and the coelomic pressure times the area of the cloaca. One would predict, then, that a net force would be available for expansion of the cloaca if the attachment area of radial muscles on the body wall exceeded the attachment area of those muscles on the cloacal wall. This is indeed the situation in cloacal breathers (Fig. 1), not surprisingly since the cloaca must be smaller in diameter than the body wall enclosing it. MATERIALS AND METHODS The maximum theoretical suction which could be developed by such a hydro- statically-supported "sac-within-a-sac" was calculated as a function of coelomic pressure and body geometry for two simplified body plans. A cylindrical body of indefinite length with a coaxial cloaca (Fig. 2), and a spherical body with a con- centric cloaca (Fig. 3), were considered as limiting cases; the geometry of living cloacal breathers lies between these extremes. Several assumptions were made to simplify calculations. The body and cloacal walls were considered infinitely com- pliant, and effects of tension in the walls therefore disregarded. Static equilibrium was assumed, avoiding frictional and other hydrodynamic effects and allowing the sum of forces across any plane of section to be set at zero. To test under more realistic conditions the relationships calculated from the mathematical models, a mechanical model with elastic ra
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
