III. Roundworms. Nematoda.—This series comprises not only lumbricoid or roundworms proper, but also threadworms. The term derives its origin from the Greek word nema, signifying a thread. It likewise includes the strongyles, the term strongulos meaning round or cylindrical. This is a very extensive group whose parasitic members are strictly entozoal, whilst the non-parasitic forms are either entirely free or they infest plants. Some of the so-called free nematoids live in the slime of animals. The artificial classification by Schneider, based on the muscular system, places these parasites in three well-marked groups, but I think it a disadvantage to separate widely many really closely allied forms. Thus, in his Polymyarii we have the genus Enstrongylus, and in his Meromyarii the Strongyli proper. Most of the genera may be fairly included in the following families:—Ascaridæ, Cheiracanthidæ, Cucullanidæ, Strongylidæ, Trichinidæ, Oxyuridæ, Trichocephalidæ, Filaridæ, Gordiidæ, Anguillulidæ.
IV. Thornheaded-worms. Acanthocephala.—This group embraces a small series of parasites, which, in general appearance, resemble the nematode worms. They differ, however, essentially, being, as the term indicates, furnished with spine-covered heads. They are, moreover, destitute of digestive organs. The species are entozoal in habit, abounding particularly in fishes and reptiles. At present, all the known forms are included in one family (Echinorhynchidæ), which also comprises only a single genus.
V. Annelid Parasites. Suctoria.—In this category one must place all such suctorial annelids as affix themselves to hosts for a longer or shorter period. Many of the leech-like parasites (Clepsinidæ, and especially Malacobdellidæ) remind one of certain flukes (Tristoma, &c.) possessing ectozoal habits; whilst the leeches, properly so called, afford instances of the passage from a semi-parasitic to what has been called the free parasitic mode of existence. In tropical countries these creatures very readily attach themselves to man and animals, often creating severe distress. The genera Clepsine and Hæmocharis attack mollusks and fishes respectively. The species are all ectoparasitic and exceedingly numerous. They cannot be described in this work.
VI. Arachnid parasites, Arachnida (part of).—The great class of articulated, limb-jointed, or, more strictly, arthropodous animals, includes a variety of parasites. The mites, true ticks, and such like creatures, belong to this group. Some few of them are entozoal in habit, others are only partially so, whilst the majority are entirely ectozoal. Of the two great sections of Arachnida, namely, Pulmonaria and Trachearia, the latter alone contains strictly parasitic forms. The parasitic species belong to the following families:—Pentastomidæ, Pycnogonidæ, Ixodidæ, Acaridæ, Gamasidæ, Hydrachnidæ, Solpugidæ. The parasitism of some of the species is very partial or slight. Thus, certain of the water mites, in their juvenile state, dwell on aquatic insects only; and the tick-like Gamasidæ occur upon dung-beetles. The other ectozoal species attack vertebrated animals, and several attach themselves to man himself. The whale lice (Cyamidæ) are here included in the Pycnogonidæ, though often placed by zoologists with the Crustaceans.
VII. Crustacean Parasites. Crustacea (part of).—A large number of species belonging to various well-marked sections of this great class of Invertebrates are parasitic in their habits, most of them being comprised in the so-called haustellated group. They are familiarly known to zoologists as Epizoa. As this latter term implies, they are strictly ectozoal in character, most of the species victimising fishes by attaching themselves, not only to the general surface of the body, but also to the eyes, and especially to the gills or branchiæ. The species for the most part belong to the families Lernæidæ, Caligidæ, Dichelestidæ, and Argulidæ. In this category must likewise be placed two other families belonging to the so-called isopodous section of edriophthalmatous crustaceans. These are the Cymothoidæ, which attach themselves to the tails of fishes, and the Bopyridæ, which occupy the branchial cavity of shrimps. The nature of this work precludes any detailed notice of the numerous members of this section.
VIII. Insect Parasites. Insecta (part of).—The insects, properly so called (that is to say, arthropodous, evertebrated creatures, with six legs), are many of them essentially parasitic in their habits. The most important of these are “bots” and other larvæ or maggots of various flies (Diptera). The varieties of lice are also included in this group. Some few of the insect parasites are strictly entozoal in habit, at least for a part of their lifetime, being previously attached externally for a short period only. Most of the forms are essentially ectozoal. A very large number of insect tormentors, although deriving nourishment from their victims, attach themselves to the animals for so short a time that they cannot be classed as parasites under the ordinary acceptation of the term. As examples of the so-called free parasitism, the autumnal flies (Tabanidæ) and Stomoxys may be cited. Although embracing but few strictly parasitic forms we have the following:—Œstridæ, Hippoboscidæ (with Melophagus), and Nycteribiidæ. In regard to the maggots of Muscidæ and Sarcophagæ, some of them are parasitic on animals and man, whilst others are parasitic upon insects themselves. The larvæ of Conopidæ attack humble-bees internally. Those parasitic insects, properly so called, which, like certain of the crustaceans, are sometimes spoken of as epizoa, comprise three well-marked families. Thus, we have Pediculidæ (the source of lousiness), Philopteridæ, and Liotheidæ. Both of the latter embrace numerous species which for the most part content themselves with devouring the feathers of birds and the hairs of quadrupeds. In addition to these it may be added that some of the rat-tailed larvæ or Helophilus maggots (Syrphidæ) are parasitic in man and quadrupeds, as are also the larvæ of the churchyard beetle (Blaptidæ). The closely allied Tenebrionidæ and other coleopterous families also supply various maggots possessed of parasitic habits. Fleas and bugs come under Van Beneden’s category of free parasites. This is equivalent to calling them non-parasitic parasites, an expression which looks very like a contradiction of terms.
IX. Protozoal Parasites. Protozoa (part of.)—This miscellaneous assemblage of minute creatures embraces a number of parasites of very low organisation. In the present work it is neither desirable nor necessary to hazard any statements respecting their precise zoological position. It is sufficient to say that the parasitic protozoa are for the most part entozoal in habit, not a few of them possessing vegetable affinities. The microscopic Bacteridæ, Gregarinidæ, and Psorospermiæ, comprise a multitude of organisms which are strictly parasitic in their habits, whilst amongst the Infusoria we find numerous forms which, though dwelling in the intestinal canal of their hosts, do not derive nourishment in a direct manner from their bearers. Of this kind are Paramecium and Balantidium. The separation of the psorospermiæ and gregarinæ into genera is attended with difficulty; nevertheless, I have for convenience long recognised various types under titles corresponding with the names of the observers who first discovered them (Hesslingia, Gubleria, Lindermannia, and so forth). Of necessity, the protozoal parasites will only be incidentally noticed in this work. In this category I place the falsely so called “cattle-plague bodies.” The micrococci and bacteria hardly come within the province of the helminthologist.
Without prejudice to the foregoing restrictions I must at the same time observe that the varied characters presented by the above-mentioned groups show how impossible it is to treat the subject of parasitism adequately, if one is obliged to confine his remarks to the internal parasites or helminths proper. Many creatures possessed of entozoal and ectozoal habits are parasites in every legitimate sense of the term, and yet they do not belong to the class Helmintha in its common zoological acceptation. That class taken by itself may still be allowed to stand pretty much as I represented it in 1864; but in the present work I cease to speak of the Entozoa as in any sense the zoological equivalent of the Helmintha. I prefer to employ the term Entozoa in its popular and wider acceptation. It conveniently stands thus, moreover, in direct contradiction to the term Ectozoa.
As this work treats of parasites only, I purposely refrain from dealing with the Turbellarians, and certain other creatures usually classed with Vermes. The vague term “worms,” so often employed as the equivalent of Helmintha, is misleading in many ways. I should like to see it adopted only when speaking of the Annelids proper. It would still have a sufficiently wide application, seeing that it would include Leeches, Earth-worms, Naids, Tubed-worms, Sea-lobworms, Sea-mice, Nereids, and a host of other setigerous species. Notwithstanding the remote connection subsisting between “intestinal worms” and worms properly so called, the notion that an intimate relation subsists between the lumbricoid helminths and earth-worms will probably never entirely disappear from the popular or even from the professional mind.