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Ascaris lumbricoides provides an example of a human parasite: the adult is an intestinal roundworm, while its infective stage is an embryonated egg acquired from a contaminated environment. The account below gives the established modern life cycle; its full migratory detail should not be assumed to have been demonstrated by December 1864.

The adult is elongated, cylindrical and unsegmented, with a tapering body, a tough cuticle and a mouth bordered by three prominent lips. Sexes are separate. The female is generally larger; the male has a characteristically curved posterior end. Their form distinguishes them from segmented tapeworms and from an earthworm, despite superficial resemblance at a distance. Adults occupy the lumen of the small intestine, obtaining nourishment in the host's intestinal environment.

Females pass large numbers of eggs into the intestinal contents, and the eggs leave in faeces. Fertilized eggs have a thick shell, commonly with an outer mammillated coat; the coat may be absent in a decorticated egg. Unfertilized eggs have a different appearance and do not develop into the infective stage. Warmth, moisture, shade and time permit larval development inside suitable fertilized eggs. Freshly passed eggs are therefore not equivalent to fully developed infective eggs.^[ref]

After an infective egg is swallowed, a larva hatches, crosses the intestinal mucosa and reaches the liver and then the lungs through the circulation. It enters the pulmonary air spaces, ascends the bronchial tree to the throat, and is swallowed. Back in the small intestine, it develops into an adult and completes the cycle. No intermediate insect, fish or mammal is required for this human roundworm cycle.^[ref] Development in the outside environment helps explain why ordinary immediate contact with newly passed faeces is not the same exposure as swallowing mature eggs from contaminated soil.

The migrating larva and the adult affect different tissues. Many light intestinal infections are asymptomatic, whereas heavy burdens can impair nutrition or obstruct the intestine; wandering adults can enter biliary passages. Larval migration can produce respiratory inflammation. These effects arise from particular stages and burdens, not from venom injected by a biting organ. Eggs in a suitable stool preparation and the morphology of an expelled adult provide different kinds of identifying evidence; an early migratory infection need not yet supply eggs in stool.^[ref]

The natural history explains prevention: separating human waste from food and water and limiting contaminated-soil ingestion interrupt transfer of eggs. It also explains why a household observation of a worm does not establish spontaneous generation inside the intestine. The absence of an intermediate host is compatible with a complex larval migration within the definitive host.

The principal intestinal Entozoa to describe are roundworms, threadworms, whipworms and tapeworms; the chief visceral larval form is the hydatid. “In this country” refers to the British setting of the paper. It is not a claim that all human helminths were common or acquired locally. A contemporary account already distinguished Nematoda, Cestoda and Trematoda, and recognised that an adult worm and a larval cyst could be different phases of one life cycle.^[ref] The life cycles below use present knowledge to explain transmission; their inclusion does not imply that every migratory stage or intermediate host was established in 1865.

Ascaris lumbricoides lives as a large, cylindrical, unsegmented, tapering roundworm in the small intestine; adults are cream-coloured or pinkish and roughly 15–35 cm long, with the male generally smaller and its tail curved. Eggs leave in faeces and become infective after development in the environment. After ingestion, larvae cross the intestinal wall, pass through the liver and lungs, ascend the airway, are swallowed and mature in the small intestine. Adults can wander into the bile ducts. The pulmonary migration belongs to the larval history, not to the usual location of the adult.^[ref]

Enterobius vermicularis, the threadworm, is a small white nematode of the caecal and adjacent colonic region. The female is about 8–13 mm long and has a conspicuous pointed tail; the male is shorter. The gravid female migrates to the perianal skin and deposits eggs, often at night, so itching and transfer by contaminated fingers favour renewed infection. Eggs become infective rapidly; a pulmonary migration is not part of its usual life cycle. This explains why a perianal adhesive sample can be more useful than an ordinary stool examination.^[ref]

Trichuris trichiura, historically Trichocephalus dispar, inhabits the caecum and colon. The adult is about 3–5 cm long, with a very slender anterior portion embedded in the mucous membrane and a thicker posterior portion projecting into the lumen: the outline resembles a whip. Eggs pass in faeces, mature in soil and infect after ingestion. The adult shape and the characteristic eggs with polar plugs distinguish it from the preceding worms.^[ref]

Adult Taenia are flat, ribbon-like tapeworms, several metres long, with a small attachment head or scolex, a narrow neck and a chain of proglottids. Taenia solium has four suckers and a hooked rostellum; Taenia saginata, also called Taenia mediocanellata in period literature, has four suckers but lacks that crown of hooks. Their larval cysticerci occur in pig and cattle tissues respectively; ingestion of inadequately cooked infected meat permits an adult to develop in the human small intestine. Eggs and segments pass in faeces. Egg appearance alone does not reliably distinguish these two species.^[ref]

For Taenia solium, swallowing eggs is a different event from swallowing pork cysticerci: eggs can establish tissue cysticercosis in humans, including abdominal tissues, whereas pork larvae cause intestinal taeniasis. A cysticercus is a small fluid-filled bladder with an invaginated scolex, not a miniature chain of segments.^[ref]

The hydatid of Echinococcus granulosus is particularly important in the liver. Humans acquire eggs from contamination associated with infected dogs and act as intermediate hosts. Larvae reach viscera and form a fluid-filled cyst with a laminated wall and an inner germinal layer; brood capsules, protoscolices and sometimes daughter cysts develop inside. The tiny adult tapeworm belongs in the dog's intestine, not as the normal human liver worm. Dogs complete the cycle by eating infected viscera from an intermediate host.^[ref]

Less usual examples deserve proportionate mention. Diphyllobothrium latum is a broad, segmented fish tapeworm with two longitudinal attachment grooves rather than four suckers; aquatic intermediate hosts and infected fish link it to humans. It should not be presented as a uniformly common British infection.^[ref] Fasciola hepatica, a leaf-shaped fluke roughly 2–3 cm long with oral and ventral suckers, can inhabit the bile ducts; its cycle requires a freshwater snail and infective stages on aquatic vegetation. Human disease was an unusual counterpart of an important infection of grazing animals.^[ref] Trichinella spiralis has minute intestinal adults after ingestion of infected meat, but its enduring encysted larvae occupy skeletal muscle; those muscle cysts should not be misdescribed as the typical abdominal hydatid.^[ref]

Gross form and location guide identification, but microscopy of eggs, larvae or segments and knowledge of the life cycle supply the confirmation.

The historical Vermes is a broad assemblage of worm-like animals; Arachnida is an arthropod group with a fundamentally different appendage-bearing plan. Vermes has no single set of characters defining an equivalent modern natural clade, so it is necessary to state the scope of the comparison.

Animals called Vermes typically have elongated or flattened bodies and lack paired jointed walking legs of the arthropod type. Many show bilateral symmetry. Their internal organization is highly variable: some possess a complete alimentary canal, some a blind-ending one, and Cestoda lack an alimentary canal entirely. Body cavities, body coverings and segmentation also vary. Modern Annelida, Nematoda and flatworms are consequently distinct groups; the segmented body of an annelid and the chain of reproductive proglottids in a tapeworm should not be identified as the same organization. It would be inaccurate to characterize all Vermes as either segmented or unsegmented, or all as having no digestive tract.

Parasitic examples within that historical assemblage include Ascaris lumbricoides, Enterobius vermicularis, Trichuris trichiura and Trichinella spiralis among Nematoda; Taenia solium and Taenia saginata among Cestoda; and Fasciola hepatica or Schistosoma haematobium among Trematoda. These illustrate intestinal adults, tissue larvae and blood- or bile-duct inhabitants rather than one obligatory habitat. Leeches provide annelid examples, including blood-feeding temporary external parasites. A parasitic mode of life alone cannot unite these forms into a natural zoological class.^[ref][ref]

An adult typical arachnid has a chitinous exoskeleton, jointed appendages, and a body organized into a prosoma and opisthosoma, though their boundaries may be inconspicuous or fused in some groups. The prosoma bears chelicerae, pedipalps and four pairs of walking legs. There are no antennae and no wings. These features distinguish the usual arachnid plan from the three pairs of adult insect walking legs and the antenna-bearing insect head. Respiration may use book lungs, arthropod tracheae, body-surface exchange or combinations according to the group; book lungs are not a universal requirement.^[ref]

The strongest parasitic examples are mites and ticks. Sarcoptes scabiei, the itch mite, burrows in the superficial skin and has eight legs in its nymphal and adult stages; its larva has six. The Demodex mites occupy hair follicles and associated glandular sites, with elongated bodies that should not make them nematode worms. Ixodes and Argas include blood-feeding ticks. Their compact or fused body regions do not abolish their arachnid appendage plan. The six-legged larva of a tick or mite is a developmental exception to the adult eight-leg character, not evidence that it becomes an insect.^[ref][ref]

A final useful negative example is the louse: although a familiar ectoparasite, it is an insect, not an arachnid. Conversely, a free-living spider is an arachnid without being a parasite. The comparison therefore requires structural characters as well as a separate specification of which members are parasitic.

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.^[ref][ref]

The leading distinctions are as follows:

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:

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.^[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.^[ref]

A crucial distinction is that 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.^[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.^{[ref][ref][ref][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.