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n.1.(Zool.) A larval entozoön in the form of a subglobular or oval vesicle, or hydatid, filled with fluid, sometimes found in the tissues of man and the lower animals; - so called from the absence of a head or visible organs on the vesicle. These cysts are the immature stages of certain tapeworms. Also applied to similar cysts of different origin.
Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
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Virchow speculated that the protoscolices may have perished in the smaller vesicles or been transformed into a cystic form, which he called acephalocysts. He stated that "there is no longer any doubt about the existence of sterile echinoccocal vesicles in human beings and it seems likely that they developed from immature, hookless [protoscolices]." We know today that in most human patients with alveolar echinococcosis, the parasitic tissue has only sterile vesicles and that protoscolices obtained from lesions of the rodent intermediate host can transform into vesicles in cell culture (11).