Effector mechanism in the response of Schistosoma mansoni miracidia to snail‐conditioned water

Schistosoma mansoni miracidia respond to snail‐conditioned water (SCW) by sharply increasing their rate of turning when they encounter abrupt decreases in stimulant concentration (Roberts et al., '79). We examined the role of the cilia and the subepithelial muscles in the turning behavior of stimulated miracidia. Several lines of evidence indicated that miracidia do not turn by altering their ciliary beat. Ciliary beating on detergent‐treated miracidia was reactivated using solutions containing ATP and Mg²⁺. These organisms were unable to turn spontaneously, a characteristic of live miracidia. Several divalent cations which stimulate increased turning of intact miracidia failed to support ciliary reactivation of detergent‐treatedor, ganisms. Further, intact miracidia increased their rate of turning in gradients of Mg²⁺, but detergent‐treated organisms swimming in reactivation solution did not turn in Mg²⁺ gradients. High speed cinematography of intact miracidia swimming in gradients of SCW or Mg²⁺ illustrated that turning is invariably accompanied by flexion of the body. This bending occurred only at the transverse interciliary ridges between the first, and second, and second and third tiers of ciliated plates. Flexion was not observed at the interciliary ridge between the third and fourth tiers of plates, suggesting that miracidia turn by contracting specific portions of their subepithelial musculature.