Past exam of the medicine course of the University of Oxford / 1866 / First examination / Anatomy and physiology / 9

What are the distinctive characters of the classes Vermes and Arachnida ? Specify parasitic members from each of these classes.

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.