Studies on the helminth fauna of Alaska. LI. Observations on eggshell formation in some diphyllobothriid cestodes

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Abstract
Formation and structure of the eggshell were studied with light and electron microscopy in 13 species of diphyllobothriid cestodes. In those having marine intermediate and final hosts, the shell of the fully developed egg is deeply pitted; the eggs as they first appear in the uterus are up to four times the size of mature eggs, and are asymmetrical, thin-shelled, cohesive, and plastic. As they move down the uterus, they become smaller, the shell thickens, the pits enlarge and deepen penetrating the shell to the lipoprotein layer, and the opercular suture develops. The pits may be formed by coalescence of proteins before quinone tanning. Hatching of eggs from such cestodes depends upon salinity rather than light; the pits may provide the means by which salt ions are conveyed to the mechanism that controls hatching. In cestodes having the aquatic phase of their life cycle in freshwater, the eggshells are only superficially pitted, and hatching usually is mediated by light. Origin of the pits in relation to eggshell formation is discussed, as is their possible phylogenetic and taxonomic significance.

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