It is a common thing to find that parasites are either hermaphrodite or that the male is degenerate, as is the case with many of the parasitic Crustacea, but with one or two exceptions the Nematoda are bisexual, and although, as a rule, the males are smaller than the females, they show no other trace of degeneracy.
In spite of the fact that the class as a whole shows but few special modifications consequent on a parasitic mode of life, it is clear that the Nematoda are peculiarly adapted for such a mode of life. Their elongated thread-like bodies afford little resistance to the passage of the food, which, as it passes through the intestine of the host, might tend to carry the parasites out of the body. At the same time their shape enables them to pierce and wriggle through the various tissues without making any very serious lesions such as might prove fatal to their host. Their extraordinary power of resisting desiccation both in the egg and in the adult state vastly increases their chances of ultimately hitting on the right host. They are capable of living in a state of suspended animation for months, and even years when dried (vide p. [136]), and of resuming their activity on being moistened.
The great faculty this group shows for living parasitically is evinced by the extraordinary variety of life-history presented by the different species. There is scarcely a stage which may not be parasitic; the eggs, the larvae, the adults are all in some cases free, in others parasitic, and in many cases first the one and then the other.
2. Occurrence and Effect of the Parasite on the Host.—Von Linstow states that the only law that can be derived inductively from the study of the life-history of Nematodes is that those that live in animals never pass through all their stages of development in the same organ; consequently, in considering the distribution of the parasites within the body of their host we have a double habitat to consider. Many forms, such as Trichina spiralis, wander from the intestine to the muscles; others, such as Filaria medinensis, from the alimentary canal to the lymphatics or blood vessels or subcutaneous tissues. Others pass from the body-cavity to the intestine, as the Mermithidae, which infest Insects, or from the stem and leaves of a plant to its flower, as in the case of Tylenchus tritici.
With regard to their occurrence in the different classes of the animal kingdom, they have been most frequently observed in Vertebrates and in Insects. They are comparatively rare in the other large divisions. Many genera are confined to certain hosts: thus Ascaris, Filaria, Trichosoma occur only in Vertebrates; Spiroptera (with one exception) in Mammals and Birds; Cucullanus in Fishes and Amphibia; Strongylus and Physaloptera in Mammals, Birds, and Reptiles; Dochmius, Pseudalius, Trichocephalus in Mammals; Dispharagus, Hystrichis, Syngamus in Birds; Nematoxys, Hedruris in Amphibia and Reptiles; Ichthyonema in Fishes; and Isacis and Mermis in Insects.
Twenty-two species have been described as parasitic in man, of which perhaps the most dangerous are Filaria medinensis, the three varieties of F. sanguinis hominis; Dochmius (Ancylostomum) duodenalis, and Trichina spiralis. The Ascaridae, as Ascaris lumbricoides and Oxyuris vermicularis, though painful, seldom cause death.
The enormous number of parasites harboured by one host is shown by the fact mentioned in Leuckart's Parasites of Man, that Nathusius[[203]] took from a single black stork 24 specimens of Filaria labiata from the lungs, 16 Syngamus trachealis from the trachea, more than 100 Spiroptera alata from the coats of the stomach, besides several hundred Trematodes belonging to several different species (see p. [63]). Even this has been surpassed in the case of a young horse, in whose body Krause found 500 Ascaris megalocephala, 190 Oxyuris curvula, several millions of Strongylus tetracanthus, 214 Sclerostomum armatum, 287 Filaria papillosa, 69 Taenia perfoliata, and 6 Cysticercus forms.
It is impossible here to enter into a full description of the destruction caused to domesticated animals and crops by the presence of these parasites; full details will be found in books dealing especially with this question, such as Neumann's Parasites and Parasitic Diseases of Domesticated Animals. A couple of cases will show how important this matter is to the farmer. Crisp estimates that Syngamus trachealis causes the death of half a million pullets in England every year, and Mégnin states that in a single pheasantry 1200 victims died daily; again, the loss of one-third the crop of beetroot is by no means uncommon when it is infested with Heterodera schachtii. These show the practical importance of what at first sight seem quite insignificant animals, and the necessity for the minutest observation, for only when we are fully acquainted with all the details of the life-history of a parasite are we in a position to successfully combat it.
Sub-Order II. Nematomorpha.
Until the last few years it has been customary to regard the Gordiidae as a family of Nematodes. Although in external appearance and life-history they closely resemble the members of this group, yet recent research has shown so many important morphological differences between them and the Nematoda, that most zoologists are now agreed in placing them in a different sub-Order, the Nematomorpha, a name first suggested by Vejdovsky.[[204]]