Significant Factors in the Plerocercoid Environment of Diphyllobothrium latum (Linn.)

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Abstract
1. Plerocercoid larval stages of Diphyllobothrium latum (Linn.), in 0·2 molar sodium chloride can tolerate temperature values between −8° and 55°C., the survival time varying from 15 minutes at 54° C. to 60 hours at 20° C., and diminishing gradually from 24+ hours at zero to non-survival at −8°C. The optimum range of values lies between 38° and −2·8° C. It is suggested that the median point of the optimum range approximates to the mean summer temperature of the host fish and that variations in temperature from this mean point restrict the survival time value of the intramuscular plerocercoid phase.2. They can tolerate concentrations of HCl between 0·3 and 0·001 molar if 0·5 per cent. of pepsin be present, and concentrations of Na2CO2 between 0·2 and 0·001 molar, when 3 per cent. of pancreatin is present, for the length of time they would normally be subject to canine or human gastric and duodenal digestion.3. They can tolerate the component salts of Ringer-Locke solution when in concentrations between 0·2 and 0·001 molar for periods of time varying with the concentration and with the chemical structure of the salt. The addition of other electrolytes to decimolar NaCl does not increase its value as a medium for the larvae.4. The behaviour of plerocercoids in electrolyte solutions of 0·2–0·001 molar concentration is essentially similar, and initial period of activity being followed by a period of contraction and latent mobility—which, however, may be omitted—passing into a period of endosmotic immobility and disintegration. It is suggested that activity is stimulated by the influence of absorbed CI' upon muscular irritability and that depression of activity is induced by absorption of the kation of neutral salts or of the whole molecule of acids or alkalies.