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In Stentor, a narrow elongate shape permits faster diffusion. Myonemes along body wall allow shape contraction to mix cell contents. Exterior circulation by cilia helps move fresh water for gas exchange, nutrients closer to body, for exchange by diffusion.

Size matters: microorganisms use simple diffusion for gas exchange Gas Exchange in Unicellular Organisms diffusion Altering shape may make diffusion uptake a shorter, faster path

Size Matters: Surface/Volume Ratio volume = 1 cm 3 volume = 8 cm 3 surface = 6 cm 2 surface = 24 cm 2 S/V=6.0 S/V=3.0 Larger organisms have less surface area relative to volume than do smaller organisms. Materials inside the larger multicellular body have less contact with surface, so something more than diffusion is needed! 1 cm unfold the cube’s surfaces 2 cm unfold the cube’s surfaces Conclusion? Circulation!

Geometry Matters: Surface/Volume Ratio volume = 8 cm 3 surface = 24 cm 2 S/V=3.0 Larger organisms depending on diffusion for gas exchange get better gas exchange if the body is thinner rather than blockier 2 cm unfold the cube’s surfaces Conclusion? volume = 8 cm 3 surface = 28 cm 2 S/V=3.5 1 cm unfold the cube’s surfaces

Unicellular animals use diffusion ©1996 Norton Presentation Maker, W. W. Norton & Company Simple aquatic multicellular animals exchange gas through skin with capillary exchange with blood system…evaginated..or invaginatedAir breathers use lungs or tracheal systems

Bristleworms (Nereis) have capillary beds in the parapodia for gas exchange ©1996 Norton Presentation Maker, W. W. Norton & Company Invaginated or Evaginated?

Mexican Axolotl Ambystoma mexicanum See Fig pg 906

Nudibranch Flabellina verrucosa

Argopecten gibbus the Calico scallop, a bivalve mollusc ©1996 Norton Presentation Maker, W. W. Norton & Company Ciliated surfaces move water across gills for gas exchange Mucilage surface traps and accumulates food particles Cilia move food-laden mucilage to mouth for digestion

Scallop respiration and feeding (shown with one valve removed) The soft tissue anatomy of the calico scallop, Argopecten gibbus, visible following removal of one of the shell valves. Key: AM - adductor muscle; G – gills; O – ovary; T – testis; L – ligament; M – mantle. The inhalant and exhalant chambers of the mantle cavity are identified as IC and EC respectively.

Exploring the incurrent and excurrent siphons of a clam: