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\b[Hydatids, in the broader nineteenth-century zoological usage, include several bladder-like larval forms of <tapeworms>; their adult and larval appearances belong to connected life cycles.] The main forms to distinguish are the <cysticercus>, <coenurus> and echinococcal <hydatid cyst>. A fluid-filled cyst is not by itself proof of a living parasite.
A <cysticercus> is a fluid bladder with a single invaginated <protoscolex>, the future attachment region of a <tapeworm>. The historical <Cysticercus cellulosae> is the larval stage of <Taenia solium>, not a separate adult species. Eggs from a human intestinal <tapeworm> carrier can be ingested by pigs, and the released embryos penetrate the gut and reach tissues where <cysticerci> develop. A human eating infected pork containing <cysticerci> can acquire an intestinal adult. Humans can also acquire tissue <cysticercosis> by ingesting eggs. \b[Egg ingestion causing tissue larvae and larval ingestion causing an intestinal adult are different routes.] The same distinction prevents the false conclusion that a bladder worm inside a person's <brain> came there as an adult from the intestine.<DPDx: Cysticercosis>[Causal Agent; Life Cycle: eggs versus pork-borne cysticerci and tissue development]{ref}
A <coenurus> bears numerous <protoscolices> on the wall of a common bladder. The familiar cerebral form in sheep, historically <Coenurus cerebralis>, belongs chiefly to the life cycle of <Taenia multiceps>. Canids carry the adult and shed eggs; sheep and other suitable herbivores develop the tissue larva. A canid eating affected tissue can then acquire adults. <Taenia serialis> produces another <coenurus> form, commonly associated with rabbits and with a different distribution of tissue lesions. Humans are accidental intermediate hosts rather than normal hosts of intestinal adults of these <coenurus>-producing species. The multiple heads on one larval bladder distinguish a <coenurus> from the ordinary one-headed <cysticercus>.<DPDx: Coenurosis>[Causal Agents; Life Cycle; Hosts; Gross coenurus specimens: common bladder with numerous protoscolices]{ref}
In cystic <echinococcosis>, the larva of the <Echinococcus granulosus> complex forms a cyst with an outer laminated parasite layer and an inner living <germinal layer of a hydatid cyst>. The host adds its own surrounding tissue reaction, which is not the same structure as the parasite wall. The <germinal layer of a hydatid cyst> can produce <brood capsules>, <protoscolices> and daughter cysts; free brood material may form hydatid sand. A cyst without visible heads, historically called an acephalocyst, need not represent a fundamentally unrelated animal. It can be an immature or sterile form, so absence of heads in one preparation is not evidence for spontaneous generation.
The mature echinococcal <tapeworm> is small and inhabits the intestine of a suitable definitive host, classically a dog or another canid. Eggs shed in faeces reach herbivorous intermediate hosts; the released six-hooked embryos cross the intestinal wall and develop in organs, especially <liver> and <lungs>. The definitive host completes the cycle by eating infected tissue. Humans acquire the larval infection accidentally by ingesting eggs and are not the usual carriers of the adult. The route is therefore not ordinary consumption of hydatid-bearing meat followed by a human adult echinococcus. Modern classification also distinguishes organisms such as <Echinococcus multilocularis>, whose branching alveolar larval growth differs from a single typical cyst. Those finer distinctions should not be projected without qualification onto every historical hydatid name.<DPDx: Echinococcosis>[Causal Agents; Life Cycle: canid adults, intermediate-host cysts, daughter cysts and modern species distinctions]{ref}
The natural-history discovery was that dramatic differences of size, habitat and body form can be stages of one organism. Feeding and transmission experiments, together with the continuity of hooks and attachment structures during development, supplied stronger evidence than resemblance between two isolated cysts. <Thomas Spencer Cobbold>'s 1864 treatment already organizes larval forms alongside their cestode relationships.<Entozoa, an Introduction to the Study of Helminthology by Thomas Spencer Cobbold>[Cestoda; particular larvae, including coenuri; separate treatment of the echinococcus and hydatids]{ref} The broader word hydatid thus needs a stage and a host attached to it. Its descriptive use for some non-parasitic cysts is not a licence to assign every cyst to one of these life cycles.
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