= Solution
\b[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.<Entozoa, an Introduction to the Study of Helminthology by Thomas Spencer Cobbold>[1864 classification and separate accounts of Trematoda, Nematoda and Cestoda]{ref}<DPDx: Ascariasis>[Causal Agents; Life Cycle]{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.<Biology 2e by Mary Ann Clark and others/28.6 Superphylum Ecdysozoa: Arthropods>[Chelicerata: Arachnida, body regions, chelicerae, pedipalps and four walking-leg pairs]{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.<DPDx: Scabies>[Causal Agents; Life Cycle: superficial burrows, three larval leg pairs and four nymphal/adult pairs]{ref}<DPDx: Ticks>[Causal Agents; Life Cycle: hard and soft ticks and developmental stages]{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.
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