ANIMAL.] PARASITISM 259 the pig. Trichodina infests Planarians. Flagellate parasites are more numerous : Ccrcomonas intcstinalis is frequently observed in choleraic affections ; Trichomonas intcstinalis and vaginalis are also described in diseases. In perhaps all invertebrates and cold-blooded vertebrates ciliate and flagellate parasites seem to occur. Acinet& are sometimes parasitic on other Infusoria. 13y far the most important group, however, are the exclusively parasitic Gregarinida. These are very widely distributed among the tissues of invertebrates, especially worms and insects, and their normal life-history is readily observed in the species infesting the tissues of the common earthworm. Their spores or 2 }SCUH - iiaviccllce are apparently closely related to the psorosperms fre quently detected in both vertebrate and invertebrate tissues, and even in the liver and hair of the human subject. 1 Dicyemida. This group contains only one entirely parasitic genus, various species of which live in the renal organs of Cephalo poda. The adult consists essentially of a simple sac of finely ciliated ectodermal cells enclosing a single elongated endodermal cell, which discharges nutritive and reproductive functions. Some have attempted to demonstrate a mesoderm. The embryos are of two kinds, nematogenic or vermiform, and rhombogenic or infusiform, differing in origin, structure, and life-history, but of still uncertain relations and import. The infusiform embryo which becomes free is of complicated structure, and probably completes its development in some new host. Some have connected the Dicyemida with such higher forms as the Rotifcra or Trematodes, and have regarded the simplicity of the adult as the result of that degeneration which is suggested both by development and habit. Haeckel, while acknowledging degeneration, regards Dicycma as a survivor of the originally simple Gastrazada from which the Metazoa have sprung. 2 Orthonectida. This group consists of a number of minute para sites, such as PJiopalura, infesting some Nemertines, Turbellarians, and Ophiuroids. Although moving in linear direction, as their name implies, they exhibit radiate structure. The ciliated and segmented ectoderm encloses an inner endodcrm layer and a central cavity which usually contains embryos. They exhibit a well- marked sexual dimorphism, the males being smaller and with fewer segments. Their position is as problematic as that of the Dicyemida they may be regarded as degraded forms allied to the Turbcllaria, T rcmatoda, or Rotatoria, or as survivors of the Gaslrezada. 3 Ccelenterata. In this group (see CORALS, HYDROZOA), while the fixed forms are frequently indebted for support to other organisms or to each other, and although such associations occasionally seem tolerably constant, true parasitism is remarkably rare. Young Narcomedusse, (Cunina) are parasitic within the mouth of Carma- riiia, and the hydroid Lafcea parasitica grows like ivy on Aglaophcnia. Similar remarks apply to the Mollusca, where, with one or two exceptions (e.g., Entoconclia miralrilis discovered by Johannes M tiller in Synapta, and another Philippine species described by Semper) parasitism is unknown. Echinodermata. There are no parasitic Echinoderms. Vcrmes. To this sub-kingdom belong the majority of parasites, but the greater groups are treated in separate articles. See NKMATOIDEA, and for Cestoids and Trematodes see TAPEWORM. Acanthocephala. This group, usually regarded as degenerate from NKMATOIDEA (q.v.), is represented by various species of Echinorhynchus. These parasites possess a muscular elongated body with a retractile proboscis armed with hooks, which serves to fix the animal to its host. Sense organs, mouth, alimentary canal, and anus are wanting ; but the muscles, nerves, and generative organs are well developed. There is a complicated subcutaneous canal system ; the sexes are distinct, and the reproduction is vivi parous. The embryo, well provided with ensheathing membranes and with hooks, is expelled with the excreta of its vertebrate host and swallowed by some Arthropod, such as Ascllus or Gammarus. There a remarkable metamorphosis takes place: the adult is formed within the body of the larva, the skin being the only part of the larva which passes over to the adult. The young Echinorhynchus finally passes with its invertebrate host into the alimentary canal of some vertebrate, e.g. , fish or bird or even pig, and there attains sexual maturity. Rotaloria. Such forms as Albcrtia, found externally on certain worms (Nais, &c.), and Balatro inside the same, are distinctly parasitic, and are not improbably differentiations of the same form. Among the Nemerteans various parasites occur, such as Ponto- Mella, Branchcllion, Piscicola, found especially on fish. The Cheztopoda are never parasitic, and but rarely commensal. The 1 See Lcuckart, Bronn s Protozoen, and article PROTOZOA. 2 E. van Beneden, Bnlletin de I Ac.ad. Roiiale de Belgique, xli. and xlii., 187C; C. 0. Whitman, Mitth. Zool. Slat. Neapel, 1882, iv. 1-89 ; Jour. Roy. Microscop. Soc., passim. 3 Giard, Jour, de VAnat. ft de la Pfit/siol., xv.. 1879 ; Comptes Rendus. Ixxxix., 1879; Quart. Jour. Microscop. Sci., vol. xx., 1880 (figure); Metschnikoff, Zool. Am., 40-43, 1870 ; Jour. Hoy. Microscop. Soc., 1881; Ztschr. f. w. Zool.. xxxv 1881 ; Jour. Roy. Microscop. Soc., 1880, p. 86. Myzostomata are probably, however, degenerate Chsetopods, repre sented by the genus Myzostoma living ectoparasitically on Crinoids. Crustacea. This group includes an immense number of forms in varying degree parasitic. The Copepoda include all grades from free-living forms to such degenerated parasites as Achtheres, Lernsea, Chondracanthus, and Argulus. Many Entomostraca are parasitic, and among the Isopoda we find such forms as Bopyrus and Cryptoniscus. Among the Cirripedia again are various grades of parasitism from some of the Lepadidae, to the ne plus ultra of degeneration the Rliiwcephala.* Insecta. Insects furnish a large proportion of ectoparasites, but comparatively few endoparasites, for very obvious reasons. The Strepsiptera, parasitic on bees, the ichneumon-flies, Flatygaster, and allied Hymenopterous forms, the Pcdiculinse, (Hemiptera) and the Mullophaga are the more important parasites. Many of the other groups also include parasitic members. See INSECTS. Arachnida. The majority of Acarina (see MITE) are parasitic, and there are many other Arachnida of similar habit. To the Arachnida the Pycnogonida and the Pentastomida are often referred. The former are parasitic in their youth at least on Hydroids. Pcntastomum exhibits considerable divergence from the Arachnoid type, and has a life-history closely parallel to that of the Cestoids. The adult form is found in the frontal sinus of the dog or wolf; the embryos pass through the nose to the exterior, and if eaten by a hare or labbit lose their investment, penetrate to the liver, encyst, and pass through a complicated series of changes, finally attaining maturity and sexuality when the flesh of the rodent is eaten by the original host. Vcrtcbrata. The Vertebrala are rarely parasitic. The best case of incipient parasitism is that of Myxine, which burrows into the codfish. With this may be compared the well-known Rfmora, which attaches itself externally to sharks, &c. Commensalism is, however, more common, many small Teleosteans living with Medusas., sea anemones, and such like. Fierasfcr finds a lodgment inside the respiratory tree of Holothurians; and Semper describes a Philippine species which actually devours the viscera of its Holo- thurian host. B. Distribution of Parasites and List of Hosts. Protozoa are of course rarely infested ; Ccclcnterata also rarely; species of Distomum have been taken on Physophora, Velella, Pclagia, Beroe, and Cesium ; a scolex and a nematoid have been described from Ctenophores, while various Arthropods occur ecto parasitically. Echinoderms are also very free from parasites ; on Echinus, however, despite its pedicellarioe, occur occasionally the semi-planariform Trematode Syndcsmis, and the molluscs Stylifcr, Anaplodium, and Eulima (the latter occurs also on starfishes). The Comcitulse, of all seas bear My~ostoma. Holothurians from the Pacific occasionally contain crustaceans, such as the crab Pinnotheres, and several Copepods. Their respiratory tree lodges Fierasfcr, while Synapta contains the mollusc Entoconclia mirabilis. Mollusca are more largely infested. Pinnotheres and other Crustacea frequently inhabit the mantle cavity of marine Lamelli- branchs, as the Arachnid Atax does the fresh-water mussel. The Lamellibranchs also have their peculiar Trematodes like Aspido- gaster and Buceplialus, besides Cercarise,, from which probablv few Gastcropods, whether marine, fresh water, or terrestrial, are ever free. The Cephalopods not only contain certain Dicycmse, in their renal organs, but through their piscivorous habits acquire Tctra- rhynchus and Ascarids. Among the Chsetopods not only are Protozoan parasites frequent, but parasitic worms are occasionally described. Crustaceans frequently contain Gregarines ; and a few Cestoids, Trematodes, and Nematoids (and Branchiobdella] have been described, as well as the cystic Echinorhynclius, from Gammarus index. More formidable, however, are the Copepods, like the familiar Nicothoe of the lobster s gills ; and, worst of all, the Rhizocephala, like Pcltogastcr and Sacculina of the hermit and shore crab respectively. Centipedes often contain Nematoids, and spiders Mermis and Gordius. Insects are preyed upon by ichneumons, are largely plagued by ticks externally, and internally by Gregarines and worms, most frequently Gordius and Mermis ; but also by larval Hymenoptera of many families, by certain Diptera, and by the Strepsiptera. See INSECTS. The Tunicata harbour many crustaceans, &c. , chiefly in the test. It is among vertebrates, however, that parasitism is most frequent and most fatal. Fishes swarm externally with Trematodes, leeches, and parasitic crustaceans, internally with cysts and intestinal worms all too numerous for enumeration. Nothing gives a more vivid idea of the extent to which parasitism has reached than an examination of a ray, or even better, the common sun fish (Orthagoriscus). Amphibians are inhabited by many parasites, the common frog 4 See CRUSTACEA, and the more recent researches of Claus and Kossmann, as also those of Lacaze-Duthiers on Laura, and the especially remarkable investiga tions of Delage on Sacculina (Zoolog. Jahresbericht, 1880-84).
