Much has been learned of the biology of cestodes since Clark Read published his paper in 1951, yet in many respects our knowledge remains distressingly meager. Read observed that, in Hymenolepis diminuta and several other species, the size attained by the worms in their definitive host was roughly inversely proportional to the number of worms present, a phenomenon he called the “crowding effect.” The adaptive value of the crowding effect seems self-evident: the size of the individual worms is limited, by whatever means, to an aggregate mass that the host can tolerate without adverse consequences. The known list of cestodes that demonstrate a crowding effect has increased little in the past 40 yr, and a definitive explanation for the effect’s mechanism of operation remains elusive. Inasmuch as development and growth of the worms is being controlled by some means external to the individuals in the...

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