Acephalocyst (Gr. a privative, head, and bladder; literally, a cyst without a head), a vesicular or hydatid growth, sometimes found in the substance of the liver, kidney, or other of the abdominal organs, in man and some of the lower animals. It is a globular bag or sac, having its walls composed of a condensed albuminous substance, of a laminated texture, and containing in its cavity a clear, colorless fluid, with albuminous or gelatinous ingredients. The main cyst produces smaller secondary cysts by a process of budding or outgrowth from its walls, and these secondary cysts are sometimes very numerous. They are developed between the layers of the principal cyst wall, and project sometimes internally and sometimes externally. Those species in which the young cysts project internally, and are thrown off into the central cavity, are called endogenous, and are found principally in the human subject; those in which they project externally are called exogenous, and are found in the ox and other ruminating animals. Acephalocysts are usually regarded as of a parasitic nature, and belonging to the class of cestoid worms, of which the ordinary tapeworm is the familiar representative.
The embryo of these cestoid worms presents at one period a globular body armed with six calcareous hooks, which afterward becomes developed into a tapeworm head, enclosed in an inverted globular membrane. When one of these partially developed tapeworm heads is found by itself, surrounded by a cyst and imbedded in one of the internal organs, it is called a cysticercus. When the principal cyst enlarges and throws off a number of secondary cysts containing tapeworm heads, it is called an echinococcus. The acephalocyst is believed to be a growth having the same origin as the above, but in which for some reason the tapeworm heads either have not been developed at all, or have become disintegrated and disappeared. Hence its name, indicating the absence of the head, which, if present, would be decisive proof of its parasitic origin.