POLYSTOSIEIE, Leuckart ; body elongate, pointed and narrow anteriorly; broad behind and generally provided with special organs of adhesion in the shape of suckers or chitinoid hooks, of suckers or claspers with chitinoid structures. Two adoral suckers In some instances. Sexual apertures median. Lamer-Stieda canal single or double. Male sexual aperture often armed with elritinoid hooks. Ova frequently provided with two long appendages.

Octobothriidm, Taselienberg.-1. Oclobolhrirrm, Nordmann; about a dozen genera of various authors are here Included by Taschenberg (30), containing fourteen species, parasitic on fishes, and almost invariably on the gills. 2. Antkocotyte, Hesse and Van Beneden ; ono species (A. merlucir), found in the bake. 3. Phylloeotyle, Hesse and Van Beneden; one species, f rein the gurnard. 4. Plat ycolyle, Ilesse and Van Beneden ; one species, from the gurnard. 5. Pleurocotyle, Gervais and Van Beneden (= Grubea cochlear, Dies.); one species, from the gills of the mackerel, 6. Diplozoon, Nordmann (see below). 7. Nexacotyle, Blainville; one species, from Thynn us brachypterus. 8. Recianocoty/e, Dies.; from the gills of Labrax 'intermit us.

Polystomidm, Van Beneden.-1. Polystomum, Zeder; two species, best known P. integerrimum (see below); Hexathyridium is probably a synonym. 2. Onchocotyle, Dies.; five species, from the gills of sharks and rays. 3. Erpocotyle, Hesse and Van Beneden ; one species, from the gills of Moste/us heris. 4. Dipiobothrium, F. S. Leuckart; one species, from the gills of a sturgeon.

Microcotylidm, Tasclienberg.-l. A rine, Abildgaard ; two species. 2. Microcotyle, Van Beneden ; about half a dozen species, all parasitic on the gills of fishes (see below). 3. Castrecoly/e, Hesse and Van Beneden; one species, from the gills of Caranx trachurus. 4. Aspidogaster, Von Baer (see below). 5. Cotylaspis, Leidy ; one species, occurring in Anodonta. 6. Aspidocotyle, Dies.

Gyrodactylidm, Van Beneden.-1. Gyrodactylus, Nordmann (see below). 2. Daelylogy•us, Dies.; about twenty species, all parasitic on fishes, mostly on the gills. 3. Tetramichus, Dies.; three species, on the gills of freshwater fishes. 4. Dipleetanum, Dies. 5. Calceostomum, Van Beneden ; one species, on the gills of Scixna aquita. 6. Sphyranura, Wright (34,45); one species, from the mouth of Menobranchus lateralls.

Ii. DIGENEA, Van Beneden ; one or more non-sexual forms Intervene between two successive sexual forms.

(i.) 3losoaxoninm, Van Beneden ; elongate, oval, or rounded in shape ; one oral sucker.-1. Monostomum, Zeder ; fifty to sixty species In mammals, birds, and fishes ; type, M. mutabile, Zeder, found in the body-cavity and eye of water-birds. 2. Notocotyle, Dies. ; N. triseria,e, Dies. ( =Ifonostomum rerrucosum).

01.) DISTOVIDA, Van Beneden ; body flattish. more or less leaf-like or elougate ; an oral and a ventral sub.median or posterior sncker.-1. Distoinum, Ttetzius (see below). 2. Fasciola, Linn.; three species are known; F. hepatica is described above ; F. gigantea inhabits the liver of the giraffe. 3. Bilharzia, Cobbold (=Cynmeophorus, Dies.); one species (see below). 4. Echinostomum, Dujardin ; E. gadorum, Van Beneden, in the intestine of Gadus carbonarius (the coal-fish), an I twenty-five other species in the alimentary canal of mammals, birds, and fishes. b. Amphistomum, Rudolph'; about twenty species in different Vertebrates ; A. subclavatum (OOze) In the rectum of the frog. 6. Gastrodiscus (?), Leuckart (see also 37). 7. Homalogasier, Poirier (38). 8. Gastrothylax, Poirier. 9. Euryccelium, Brock (44), has the excretory vessels dilated Into wide chambers (? coelom).

(Ili.) GASTEROSTOMID/E, Von Siebold ; oral sucker sub-median and ventral; also an anterior sucker.-1. Gasterostomwn, Von Slebold ; eight species, all in fishes; larval form Bucephalus (see below).

(Iv.) HOLOSTOMID/E, Clans (43); body flattened, and divided into an anterior and posterior part, the former bearing an anterior and ventral sucker; two adoral lobes with glands In connexion, or a circumoral fold with lobes.-1. Holostomum, Nitzsche; twenty-three species, most In water-birds; Il. tariabile, in various raptorial birds; larval forms Tel and Diplostomum. 2, Ifemistomum, Dies.; three species, one in the wild-cat, two in birds. 3. Eustemma, Dies.

The true position of the following is doubtful :-Nematobahrium, Van Beneden (9); Didymozoon, Von Llnstow (30); Btichocotyle, Cunningham (39). Diplozoon paradoxum (18) infests the gill of the minnow in large numbers. The eggs hatch in the water, continuing to be attached to the gill by a filament at

one extremity. The embryo is elongated oval in shape, and ciliated all over ; on its back are two eyes, consisting of a cup-shaped mass of pigment, with a spheroidal lenticular body. It presents also the mouth with two peculiar suckers, the oesophagus and intestine, and the two claspem of the Diporpa. The embryo swims vigorously about until it finds its way to the gill of a minnow, failing which It dies in about six hours. Attached to its host it may lire Isolated for a considerable time, increasing in size; usually, however, it unites with another individual In a kind of reciprocal copulation (fig. 3, A). One individual by means of Its ventral sucker seizes the dorsal papilla of another, and then the two twist across each other no that the sucker of the second seizes the papilla of the first. After this a complete fusion of the individuals takes place, the papillm and suckers growing together so firmly as to be anatomically Inseparable. Both individuals continue to grow and develop a second, third, and sometimes a fourth pair of elaspers. In Polystomum integerrimum (18), which inhabits the bladder of the frog, the eggs are developed during the winter and are laid in the spring, when the frogs resort to the water. It appears probable that the worm protrudes its body from the frog and thus deposits the egg directly in the water. The young worm, as it escapes from the egg, which takes place after a lapse of six or eight weeks, measures about 0•3 mm. in length, and swims vigorously about by the aid of a coating of cilia. At its posterior extremity is a rounded disk (fig. 3, 13_, round the margin of which sixteen delicate hooks are placed at equal intervals. Above the four hindmost of these are two others still smaller and more delicate. Upon the back ore situated four eyes disposed in pairs. The mouth is wide and leads Into a pharynx, and this into the Intestine; two excretory vessels are present, but there is no trace of generative organs. The hindermost pair of suckers is the first to be developed, and they enclose those two hooks which lie at the outer aide of the very delicate ones mentioned above, which eventually become the strong terminal hooks of the adult. The other two pairs of suckers are formed in a similar manner, the development of all three being usually completed during the month of July. Tile young Polystomum attacks not the full-grown frog but the tadpole, entering the gill-cavity and subsequently proceeding to the bladder. Like the frog It requires four or fire years to attain sexual maturity. In certain cases the Polystomum does not migrate; it then becomes prematurely sexual and dies when the tadpole undergoes metamorphosis ; under these circumstances the sexual organs are simpler than usual: the testis is simple; the germarinm Is long and coiled; there Is neither prostate nor Laurer-Stleda canal; and the oviduct has no dilated anterior portion.

Microcotyle mormyri, Lorenz (21) (fig. 3, C), has no penis, the semen issuing by an opening posterior to the spiked birth-opening; the vagina opens medially, not marginally. The posterior extremity is pointed. Arinebelones, Abildg., resembles it, but is broad and obliquely truncated behind, this margin bearing a row of fifty to seventy peculiar attaching organs, which are of the form of a hand-satchel (fig. 3, D, E), the metal clasp being represented by a complicated ehitinous framework. There are four groups of hooks and one ring of the same round the genital opening. The mouth has a sucker at either side, and above it an oval body which can be evaginated like the proboscis of a Dendrocele Planarian.

Aspidogaster conchtcola (32) is found in the pericardial cavity of the freshwater mussel; it is conical anteriorly with a terminal oral sucker; the ventral sucker is very large and divided into rectangular areas ; the excretory pore is at the posterior extremity of the body, and the genital organs open on the left side of the fore-part of tire animal (flg. 8, F).

Gyrodactylus elegans (33) is found on the fins and surface of the body of the pike, stickleback, and other freshwater fishes, and measures about 0'5 mm. in length ; it is flattened in form and tapers towards either end (fig. 3, G). At tire anterior extremity are two lappets, while the posterior is furnished with a sub-triangular plate, which bears the organs of attachment in the shape of two large curved hooks in its centre and sixteen smaller ones on its circumference. The most interesting peculiarity, however, of this farm is to be found in the fact that each embryo before it is extruded (the animal is viviparous) contains another embryo, and this in Its turn another, as that three embryonic generations are present simultaneously.

The genus Distomum is by far the most extensive In the group, containing at the present time over 300 species, which occur almost exclusively in Vertebrates; the most important are perhaps D. lanceolatum, which inhabits the same situations as Fasciola hepatica, D. clavigerum, from the frog, and D. militare, Van Beneden, from the intestine of the duck. The name Fasciola (Linn.) has the priority over Distomum (Distoma) of Retsius, which, however, has obtained extensive currency. The name Fasciola may appropriately be used In a restricted sense for forms which have a branched digestive tract, F. hepatica above described being taken as the type. This separation has not met with general recognition, although supported by Blanchard and Cobbold (1). Weiniand has proposed to substitute the term Dicroccelium, used by Dnjardin, for Distomum, retaining Fasciola for the type-species, but this proposal has not met with acceptance. The Distomes vary in size from forms almost microscopic to those which, like D. ingens, Monies (35), measure 6 cm. long in alcohol, or even 12'5 cm. in the fresh condition (D. gigas, Nardo). Distomum hafosauri, Bell (36), N parasitic upon a deep-sea fish taken in 1090 fathoms. Distomum ntacrostomuni (flg. 4, B) of the woodpecker (Apternus tridact ylus) has a remarkable larval form known as Deuce. chloridiuna paradoxunr, which is parasitic on Succinea plants, and consists of a number of branching threads, from which are developed one or two contractile sacs (fig. 4, C, D). Their growth distends the tentacle until it bursts and the sac

hangs outwards. The threads within the Succinea contain only granular cells, whilst the contractile sac is occupied by an organism ovoid In form, with a thick clear border, the rudiments of two suckers, a digestive tract, and excretory system. Bilharzia hmmatobia, Cobbold (1), is one of the most dangerous human parasites, and occurs in the blood of the portal vein and in the veins of the mesentery and bladder. The sexes are distinct, the female being from 16 to 20 mm. In length, and somewhat resembling a Nematode on superficial examination. The male is only from 10 to 14 nn. in length, but much thicker. The surface of the female is covered with fine spines most distinct towards the tail ; at the anterior pointed extremity is the oral sticker, from which a narrow opening leads into a wide pharynx, followed by the intestine, which at first forms two branches, these again uniting posteriorly to the generative organs. The ventral sucker is placed only 0-2 mm. behind the oral one, and it is immediately succeeded by a long narrow groove, which extends down the ventral surface and corresponds to the canal's gynaecophorus of the male (see fig. 4, A).

The stages in the life-history of Gasterostomuna are so remarkable that a short account of them most be given. From tire egg there escapes (1) a club-shaped embryo, which in a manner hitherto unobserved enters the freshwater mussel, Anodonta or 1772i0, where it forms (2) the eperoeyst (fig. 4, F); this Is several centimetres long and provided with lateral branches ; it occurs chiefly in the liver and ovary, and It is best developed near the skin. The wall of the sporocyst consists of cells, muscles, and perhaps a cuticle ; the extremities are pointed and filled with cells, and it is here that growth takes place. Within moniliform dilatations of these ramified tubes are formed balls of cells, each of which develops into (3) a " bucephalus." This organism (fig. 4, E) consists of a small oval body about 0'25 mm. in length with a double tail. At one extremity is a mass of glands with an invagination of the integument, which has been mistaken by many observers far the alimentary canal. This, however, opens about the middle of the body, and consists of a muscular pharynx, a forwardly directed (esophagus, and a simple saccular intestine. Tire excretory system terminates in an S-shaped vesicle, which opens posteriorly in such a way that Its contraction drives tire fluid into the tail, whence it probably finds its exit by osmosis. Traces of genital organs are found in the form of an elongated plug of cells in the hinder fourth of the body, and two rounded masses of undifferentiated cells situated dorsally. The tail is double, and from about 0•5 to 2•5 mm. long according to its state of contraction. Each half consists of a spheroidal basal portion, and an elongated tapering filament. These caudal appendages contain many nucleated connective-tissue cells with fine protoplasmic processes. The larvm swim freely in the water, but sink and perish after about twelve hours, unless they enter the mouth of certain fishes (e.g., Levelseas erythrophthatmus), when (4) they lose their tails and become encapsuled under the skin. The generative organs now become further developed; cuticular spines and the anterior sucker arc formed. If the fish thus Infected be swallowed by a pike or perch the cyst is dissolved, and the worms (5) become adult, continue to live in the intestine, and produce eggs. For further details, ace Ziegler (12).

Phylogenetic Relations.-The detailed comparisons of Van Beneden (9) and the subsequent researches of others leave no doubt that the Trematorla are closely related to the Cestoda. A consideration of their peculiarities leads moreover to the opinion that the former are more primitive than the latter; that is to say, the common ancestors of the two groups resembled Trematodes rather than Cestodes. The ancestry of the Trematoda is probably to be sought in types such as the Planarians rather than the Leeches ; characters uniting them with the former are the possession of a commonly branched alimentary canal without an alms, hat with a powerful pharynx ; the generative system is hermaphrodite, and similar arrangements obtains in the excretory vessels, nerves, and muscles, while histological agreements also are not wanting. Furthermore, certain forms are known which help to fill up the gap between the two groups : Monocelis candatus has a discoid posterior organ of attachment, and M. protraetilis a true sucker. Such forms as these could scarcely be distinguished from ectoparasitic Trematodes except for their ciliated epithelium (Leuckart, 1). In this connexion, however, it is worth while to call attention to the researches of Fewkes (40) on a marine cercania, which had a tail distinctly annelid in character, with bundles of bristles disposed at intervals along it. Compare also Schauinsland (41).

Bibliography.-(1) Jackson, in Rolleston's Forms of Animal Life, 2d ed., Oxford, 1887; other important text-hooka are Leuckart, Parasites des Atensehen, Leipsic, 1863, new edition and English translation in preparation ; Kliehenmeister and Ziirn, Parasiten des lifenschen, Leipsic, 1881 ; Cobbold, Entozoa, London, 1864; Id., Parasites, 1879; (2) Rudolph', Entozoorum Hist. Nat., Amsterdam, 1808-9, and Entozoorum Synopsis, Berlin, 1819 ; (3) Curler, Regne animal, Paris, 1829; (4) Von Slebeld, Archiv f. Naturgesch,,i., 1835; (5) Von Baer, Nova Acta Acad. Coes. Loop., xlli., 1826; (6) Steenstrup, Alternation of Generations, Ray Society, 8v o, London, 1845 ; (7) Pagensteeller, Trematodenla•ren and Trernatoden, Heidelberg, 1857; (8) Diesing, Systema Ileltninthum, Vienna, 1850; (9) Van Beneden, "Vera intestinaux," Comptes Rendus, Paris. 1861; (10) Sommer. Ztschr. wise. Zool., xxxiv., 1880; (11) Norbert, Ardli V f. mikr. Anat., xix., 1881; (12) Ziegler, Ztschr. sass. Zool., xxxix., 1883; (13) Schwarze, Op. cit., xliii., 1886; (14) Metschnlkoff, Quart. Jour. Mier Sci., xxiv., 1884; (15) Fraipont, Arch. d. Biol., 1., 1880, Ii.. 1881; (16) Plainer. Arb. Zoo!. Inst. Wien, iii., 1880; (17) Van Bencden and Lankester, Zool. Ans., iv., 1881, v. 1882; (18) Zeller, Ztschr. visa. Zool., xxii., 1872, soca, 1876; (19) Polder, Arch. Zool. Expir., HI., 1885; (20) Loose, Ztschr. ides. Amt., xli., 1885; (21) Lorenz, Arb. Zool. Inst. Wien, i., 1878; (22) Stieda, P.elehert, and Du Bois Reymond'a Archie, 1881 ; (23) Zad4ach, Zoo!. Ana., iv., 1881; (24) Lang, Mitch. Zool. Stat. Yeapel, 1881; (25) Thomas, Quart. Jour. Mier. Sci., :mill., 1883, also Jour. Roy. Agric. Soc., xix., 1883; (26) Leuckart, Zoo!. Ana, v., 1882; (27) Jackson, Op. cit., vi., 1883: (28) Sonaino, Arch. ital. Biol., vi., 1885; (29) Ercolani, Mean. Acad. Bei. Inst. Bologna, [4], 1882, iii., 1883, also abstract, Arch. Ital. Stol.,1., 1882; (30) Taschenberg, Ztschr. geaammt. Nature., Iii., 1879 ; (31) Wietzejski, Ztschr. visa. Zool., xxix., 1877 ; (32) Huxley, Anat, In rent. Anim., London, 1877; (33) Wagoner, Arch. f. Anat. v. Phys., 1860; (34) Wright, Proc. Canad. Inst., 1., 1884; (35) 5foniez, Bull. Soc. Zool. France, xi., 1880 ; (36) Bell, Ann. and Mag. Not. Fist., [5], xix., 1887 • (37) Lejtenyi, Abhandl. Sena'. Gesellsch., xii., 1821 ; (38) Poirier, Bull. Soc. Philom. Paris, (7), vit., 1883; (39) Cunningham, Trans. Roy. Soc. Ellin., xxxii., 1884; (40) Fewkes, Amer. Jour. Sci., (3), xxiii., 1882; (41) Schaninsland, Jena Zdtschr., xvi., 1883 ; (42) Pottier, Arch. Zool. Expe'r., [2], v., 1887 ; (43) Von Linstow, Archly f. Naturgesch., xllii., 1877; (44) Brock, Gattinger Nachrichten, 1887; (45) Wright and Macallum, Amer. Jour. Morph., L, 1887. (W. E. HO.)