654 THE ANIMAL KINGDOM. 



single or double. Male sexual aperture often armed with chitinoid hooks. Ova 

 frequently provided with two long appendages. 



Four sub-families : Octobothriidae, with Octobothrium, Diplozoon, &c. ; Poly- 

 stomidae, with Polystomum, Onchocotyle, &c. ; Microcotylidae, with Axine, Microcotyle, 

 &c., Aspidogaster, Cotylaspis; Gyrodactylidae, with Gyrodactylus, Dactylogyrus, 

 Calceostoma, &c. 



2. Digenetic Trematoda ; one or more non-sexual and dimorphic forms inter- 

 vene between two successive sexual forms. 



(a) Monostomidae : elongate, oval, or rounded in shape; 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, i, 1879. 

 Nematobothrium filarina attains a length of one metre, and is found encysted 

 beneath the skin of the branchial cavity of Stiaena 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. i.) 1863, p. 448 et seqq. 



List of Parasites and Hosts, von Linstow, Compendium der Helminthologie, 

 Hannover, 1878. 



Tristomeae and Polystomeae : Genera, 6<r., Taschenberg, Zeitschrift f. ges. 

 Naturwissenschaften (Giebel), 51, 1878; 52, 1879. Marine Trematoda, P. J. van 

 Beneden and Hesse, Me'm. de 1'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. 

 1860. 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, &c., Z. W. Z. xli. 1885; Poirier, Contri- 

 butions, &c., A. Z. Expt. (2), iii. 1885. Dist. cirrigerum and D. isostomum, Zaddach, 

 Z. A. iv. 1 88 1. D. Westermanni, Kerbert, A. M. A. xix. 1881. D, crassum, 



