Four sub-families: Octobothriidae, with Octobothrium, Diplozoon, etc.; Poly-stomidae, with Polystomum, Onchocotyle, etc.; Microcotylidae, with Axine, Microcotyle, etc., Aspidogaster, Coty/aspis; Gyrodactylidae, with Gyrodactylus, Dactylogyrus, Calceostoma, etc.

2. Digenetic Trematoda; one or more non-sexual and dimorphic forms intervene between two successive sexual forms.

(a) Monostomidae: elongate, oval, or rounded in shap\e; an oral sucker. Monostomum.

(b) Distomidae: body flattish, more or less leaf-like, or elongate; an oral and a ventral sub-median or posterior sucker. Distomum, Fasciola, Bilharzia, Echi-nostomum, Amphistomum, Gastrodiscus (?).

(c) Gasterostomidae: oral sucker sub-median and ventral; an anterior sucker as well. Gasterostomum, with the larval form Bucephalus.

(d) Holostomidae: body flattened, and divided into an anterior and posterior part, the former bearing an oral and ventral sucker. Two adoral lobes with glands in connection, or a circumoral fold with lobes, Holostomum, with the larval forms Tetracotyle and Diplostomum; fold incomplete, Hemistomum.

There are two remarkable genera of uncertain position, Didymozoon and Nematobothrium. The first-named occurs encysted in various fish; two individuals in one cyst, and sometimes fused posteriorly; see von Linstow, A. N. 52, 1, 1879. Nematobothrium filarina attains a length of one metre, and is found encysted beneath the skin of the branchial cavity of Sciaena Aquila. See P.J. Van Beneden, C. R. Suppl. ii. 1861, p. 107. Its-embryo has a cephalic disc armed with spines; E. Van Beneden, Q. J. M. x. 1870.

Vers Intestinaux, P. J. van Beneden, Paris, 1858 (C. R. Suppl. ii. 1861). Parasites, Cobbold, 1879. Trematodes, Leuckart, 'Parasiten des Menschen,' i. (ed. 1.) 1863, p. 448 et seqq.

List of Parasites and Hosts, von Linstow, Compendium der Helminthologie, Hannover, 1878.

Tristomeae and Polystomeae: Genera, etc, Taschenberg, Zeitschrift f. ges. Naturwissenschaften (Giebel), 51, 1878; 52, 1879. Marine Trematoda, P. J. van Beneden and Hesse, Mem. de l'Acad. Roy. Belgique, xxxiv. 1864, p. 60. Tri-stomum, Taschenberg, Abhandl. Natf. Gesellschaft zu Halle, xiv. 1877-79. Pseudo-cotyle, Id. Festscrift of same, 1879. Calicotyle, Wierzejski, Z. W. Z. xxix. 1877. Diplozoon and Diporpa, Zeller, Z. W. Z. xxii. 1872; Paulson, Mem. Acad. Imp. Sci. St. Petersburg (7), iv. 1862. Polystomum, Zeller, Z. W. Z. xxii. 1872; xxvii. 1876. Onchocotyle, Taschenberg, Festschrift (supra). Axine and Microcotyle, Lorenz, Arb. Zool. Inst. Wien, I, 1878. Aspidogaster, cf. Huxley, 'Anatomy of Inverte-brated Animals,' 1877, p. 194. Gyrodactylus, Wagener, Arch. Anat. and Physiol. i860. Stichocotyle, Cunningham, Tr. Roy. Soc. Edinburgh, xxxii. pt. 2, Session 1883-84.

Digenetic Trematoda: Synopsis of, Cobbold, J. L. S. v. 1861; Diesing, Systema Helminthum, i. 1850; cf. Id. SB. Akad. Wien, xv. 1855; xxxi. and xxxii. 1858; xxxv. 1859. Anatomy, Looss, Beitrage, etc, Z. W. Z. xli. 1885; Poirier, Contributions, etc, A. Z. Expt. (2), iii. 1885. Dist. cirrigerum and D. isostomum, Zaddach, Z. A. iv. 1881. D. Westermanni, Kerbert, A. M. A. xix. 1881. D. crassum, Cobbold, J. L. S. xii. 1876. D. Ringeri, Manson, On the Filaria sanguinis hominis, etc, Lewis, London, 1883, cap. vii. p. 134. Fasciola hepatica, Mace*, 'Recherches anat. sur la grande douve du foie,' Paris, 1882; Sommer, Z. W. Z. xxxiv. 1880. Bilharzia, Fritsch, Z. A. viii. 1885, and Leuckart, op. cit. supra, p. 617. Gastro-discus, Lejtdnyi, Abhandl. Senck. Gesellschaft, xii. 1881. Amphistomum, Blumberg, Inaug. diss. Dorpat, 1871. Opisthotrema, Fischer, Z. W. Z. xl. 1884. Gastero-stomum and Bucephalus, Ziegler, Z. W. Z. xxxix. 1883. Holostomum with Diplo-stomum and Tetracotyle, von Linstow, A. N. 43 (I), 1877, p. 187 et seqq.

Typical Life-history with anatomy, Thomas, 'On the Liver Fluke,' Q. J. M. xxiii. 1883; cf. Id. Royal Agricultural Soc. Journal (2), 17, 1881; 18, 1882; 19, 1883; Nature, xxvi. 1882; Leuckart, Z. A. iv. 1881; v. 1882; A. N. 48, 1882; W. H. Jackson, Z. A. vi. 1883. Life-histories and non-sexual forms, P. J. van Beneden, supra; Ercolani, Dell' adattamento delle specie all' ambiente, Mem. Accad. Sci. Istit. Lomb. Bologna (4), ii. 1882; iii. 1883 (cf. Arch. Ital. de Biologie, i. 1882, for abstract with plate); Moulinie, De la Reproduction chez lesTrematodes endoparasites, Mem. de l'lnstitut National Genevois, iii. 1855; de' Filippi, Mem. Accad. Torino (2), xv. 1855; xvi. 1857; xviii. 1859 (cf. A. Sc. N. (4), ii. 1854; iii. 1855; vi. 1856).

Formation of embryo, Schauinsland, J. Z. xvi. 1883; embryo of Bilharzia, Chatin, A. Sc. N. (6), xi. 1881. Synoptic tables of larvae, von Willemoes-Suhm, Z. W. Z. xxiii. 1873, p. 341-3. Anatomy of Sporocyst, Biehringer, Arb. Zool. Zoot. Inst. Wurzburg, vii. 1885; of Redia, Thomas, supra. Leucochloridium (= Macro-stomum), Zeller, Z. W. Z. xxiv. 1874. Bucephalus (sporocyst) = Gasterostomum, de Lacaze-Duthiers, A. Sc. N. (4), i. 1854; McCrady, Proc. Boston Soc. xvi. 1874; cf. Ziegler, Z. W. Z. xxxix. 1883, pp. 539-40. Cercaria: Postembryonal development of Trematoda, Schwarze, Z. W. Z. xliii. 1886. Cercariae with spines and bifid tails, see Villot, 'Marine Endoparasitic Trematodes,' A. Sc. N. (6), viii. 1879; Fewkes, American Journal of Science (3), xxiii. 1882. Bod-containing cells of Cercaria, Thomas, Q. J. M. xxiii. p. 127; Sonsino, Archives Ital. Biol. vi. 1885.

Summary of views on nature of cuticle, Looss, Z. W. Z. xli. p. 391 et seqq.; Ziegler, ibid, xxxix. p. 543 et seqq.; see also Biehringer and Schwarze, supra. •

Nervous system: Gaffron, on Dist. isostomum, Schneider's Zool. Beitrage, i. pt. 2, 1884; Lang on Tristomum, etc, Mitth. Zool. Stat. Naples, ii. 1881; and Poirier, op. cit. supra.

Excretory system, Fraipont, Archives de Biologie, i. 1880; ii. 1881; and Looss, op. cit. supra.

Fertilisation, cf. note 2, p. 648, ante.