= Solution
\b[<Entozoa> is a historical grouping of internal animal <parasites>, especially parasitic worms; it is not a single natural modern <clade>.] The core human examples belong principally to the <flatworms>, including <Cestoda> and <Trematoda>, and to the <roundworms>, or <Nematoda>. The distinction is structural and developmental, not merely between inhabitants of the <intestine> and inhabitants of other organs.
The nineteenth-century higher arrangement was unsettled. Schemes placed these animals within <Vermes>, or within annuloid assemblages that included other supposedly related simple invertebrates. For a specifically pre-exam example, the 1865 geological-museum classification put <Entozoa> in <Annuloida>, within <Annulosa>. These historical groups must not be equated with the modern <Annelida>: an <Ascaris lumbricoides> is not an earthworm. A more useful subdivision for this answer is the four orders treated in <Thomas Spencer Cobbold>'s 1864 <Entozoa, an Introduction to the Study of Helminthology by Thomas Spencer Cobbold>: <Trematoda>, <Nematoda>, <Acanthocephala> and <Cestoda>.<Catalogue of the Collection of Fossils in the Museum of Practical Geology (1865)>[p. lxxvi, classification table: Annulosa, Annuloida, Entozoa]{ref}<Entozoa, an Introduction to the Study of Helminthology by Thomas Spencer Cobbold>[contents and order headings: Trematoda p. 14, Nematoda, Acanthocephala p. 97, Cestoda p. 104]{ref}
The leading distinctions are as follows:
* <Cestoda>, the <tapeworms>, have a flattened adult body with an attachment <scolex> and, in the familiar human forms, a chain of reproductive <proglottids>. They lack an alimentary canal and absorb nutrients across the <tegument>. The <proglottids> are repeated reproductive units, not annelid segments. Typical development passes through an egg and one or more larval stages, commonly in an <intermediate host>, before the adult reaches a <definitive host>. A bladder-like <cysticercus> and its adult <tapeworm> are stages of one life cycle.
* <Trematoda>, the <flukes>, have flattened unsegmented bodies, attachment suckers and a mouth leading to a usually branched, blind-ending <digestive system>. Most are <hermaphrodites>, but <Schistosoma> has separate sexes. Typical human-infecting <flukes> use a molluscan <intermediate host>, undergo multiplication in larval stages there, and produce dispersing <cercariae>. The route from that stage into the human varies by species; there is no universal requirement to swallow an adult worm.
* <Nematoda> have cylindrical unsegmented bodies, a resistant <cuticle>, a fluid-filled <pseudocoelom>, longitudinal <muscles> and a complete alimentary canal with mouth and anus. The sexes are usually separate. Development includes <moults>; some life cycles are direct, while others need a vector or another host. Human infection may follow swallowing infective eggs, penetration by larvae, ingestion of tissue larvae or transmission by a vector, depending on the species.
* <Acanthocephala>, the thorny-headed worms, possess a retractile hooked <proboscis> and lack an alimentary canal. Their development normally involves an arthropod <intermediate host>. They belong in a general survey of historical <Entozoa>, but are not among the usual common human intestinal worms.
For a British medical answer of this period, the ordinary intestinal list should begin with the following species; the modern life-cycle details clarify their identities rather than claiming that every step was already established in 1867:
* <Ascaris lumbricoides>, the large intestinal <roundworm>. Adults inhabit the <small intestine>; infective eggs are acquired from the environment, and larvae migrate through the <liver> and <lungs> before returning to mature in the <intestine>.<DPDx: Ascariasis>[Causal Agents; Life Cycle]{ref}
* <Enterobius vermicularis>, historically commonly named <Oxyuris vermicularis>, the small pinworm or threadworm. Adults chiefly occupy the caecal region; females deposit eggs on the perianal <skin>, explaining spread through contaminated hands and objects and the usefulness of perianal sampling rather than reliance on ordinary stool samples.<DPDx: Enterobiasis>[Causal Agent; Life Cycle; Laboratory Diagnosis]{ref}
* <Trichuris trichiura>, historically <Trichocephalus dispar>, the whipworm. The thin anterior end is embedded in the caecal or colonic mucosa; eggs passed in faeces must develop outside the host before becoming infective.<DPDx: Trichuriasis>[Causal Agent; Life Cycle]{ref}
* <Taenia solium>, the pork <tapeworm>, with an armed <scolex>, and <Taenia saginata>, the beef <tapeworm>, historically often <Taenia mediocanellata>, with an unarmed <scolex>. Intestinal adults follow ingestion of tissue <cysticerci> in infected meat. Their eggs alone cannot reliably distinguish the two species by ordinary <microscopy>.<DPDx: Taeniasis>[Causal Agents; Life Cycle; Taenia spp. scoleces; Laboratory Diagnosis]{ref}
* The broad fish <tapeworm>, historically <Bothriocephalus latus>, later <Diphyllobothrium latum>, now <Dibothriocephalus latus>. It is especially relevant in freshwater-fish-eating populations and has a life cycle involving a copepod and fish before the human definitive stage. Its attachment organs are longitudinal grooves, rather than the four rounded suckers typical of <Taenia>.<DPDx: Diphyllobothriasis>[Causal Agents; Life Cycle]{ref}
Important tissue examples extend the answer beyond intestinal adults. The historical <Trichina spiralis> is <Trichinella spiralis>: ingested infected meat carries larvae, adults develop in the <small intestine>, and new larvae migrate to <skeletal muscle>. This is a distinct developmental pattern from the egg-acquired intestinal <Ascaris lumbricoides> infection.<DPDx: Trichinellosis>[Causal Agents; Life Cycle]{ref} Human hydatids are the larval <hydatid cysts> of <Echinococcus>, classically the <Echinococcus granulosus> complex; the small adult <tapeworm> is in a canid, while humans accidentally occupy the <intermediate host> role, commonly with <liver> or <lung> cysts.<DPDx: Echinococcosis>[Causal Agents; Life Cycle, Cystic Echinococcosis]{ref}
A crucial distinction is that \b[human <Taenia solium> <cysticercosis> follows ingestion of eggs; eating pork containing <cysticerci> produces intestinal <taeniasis>]. The historical name <Cysticercus cellulosae> designates a larval form, rather than an additional independent species to add to the list.<DPDx: Cysticercosis>[Causal Agent; Life Cycle]{ref}
Geographical qualification matters: <Ancylostoma duodenale>, <Dracunculus medinensis> and <Schistosoma haematobium> are important in their endemic settings; <Fasciola hepatica> is a familiar liver <fluke> but is not an ordinary common human worm in Britain. An enumeration should not turn this into a claim that all named species are equally common in every population.<DPDx: Intestinal Hookworm>[Causal Agents; Geographic Distribution]{ref}<DPDx: Dracunculiasis>[Causal Agent; Geographic Distribution]{ref}<DPDx: Schistosomiasis Infection>[Causal Agents; Geographic Distribution]{ref}<DPDx: Fascioliasis>[Causal Agents; Geographic Distribution]{ref} Conversely, calling only the familiar intestinal adults <Entozoa> would omit the instructive tissue stages and their transmission.
The decisive experimental insight is continuity of development. Comparing eggs, tissue larvae and adults, and tracing transfer between appropriate hosts, demonstrates a life cycle rather than spontaneous generation within a patient. In diagnostic material, an adult attachment organ, a <proglottid>, an egg and a tissue cyst answer different identification questions. Identification therefore depends on the stage and the material actually supplied.
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