648 THE ANIMAL KINGDOM. 



Beneden, or ' uterus ' by Taschenberg. A germ, a certain quantity of 

 vitelline cells and spermatozoa are moulded together in it into an egg, 

 and provided with a shell. The ovum is then passed on. A number 

 of ova may accumulate in the terminal portion of the oviduct which is 

 then dilated, or each ovum is laid singly as soon as formed, e. g. in 

 Epibdella, Calceostoma. The ootype is not represented in the digenetic 

 Trematoda with perhaps the exception of Gasterostomum, but the oviduct 

 is dilated, thrown into convolutions which occupy a greater or less extent 

 of the body, and retain a large number of ova. In some Distomidae, e. g. 

 D. clavatum, the terminal part of the oviduct is surrounded by unicellular 

 glands. The oviducal aperture may in some monogenetic Trematoda, 

 e. g. Axine, Microcotyle, be surrounded by chitinoid hooks. The male 

 and female apertures are always close together. They are usually placed 

 anteriorly on the ventral surface a little to one side, e. g. Tristomum, or 

 medianly, and in the Distomidae as a rule in front of the ventral sucker. 

 They are, however, sometimes placed posteriorly to it, e.g. D.ocreatum, &c., 

 or even on the posterior margin of the body as in D. macrostoimim, 

 Opisthotrema, Holostomum ; rarely on the lateral margin as in D. clavi- 

 gerum from the frog and the Tristomidan Epibdella. In many Distomidae 

 the two orifices are surrounded by a sexual cloaca, and the male orifice 

 may be situated on a projecting muscular papilla which bears in some 

 instances the female aperture as well 1 . 



Self-impregnation appears to occur as well as reciprocal impreg- 

 nation 2 . The ovum has a shell at first clear, then becoming coloured, 



1 The prematurely sexual form of Polystomum integerrimum has sexual organs simplified as 

 compared with those of the usual form. The testis is single, globular ; and the vas deferens without 

 prostatic glands. The germarium is long, coiled; the vitellarium of restricted extent; the two 

 Laurer-Stieda canals absent ; the oviduct devoid of a terminal coiled and dilated portion. Each 

 ovum is laid as soon as formed. The animal is probably self-fertilising. Zeller, Z. W. Z. xxvii. 

 1876. 



3 Reciprocal impregnation has been observed by Zeller in P. integerrimum, by Looss in 

 D. davigerum (Z. W. Z. xli. 1885, p. 426), by Cobbold in Campula (= D.campuld). The presence 

 of hooks round the female apertures of some monogenetic Trematoda points in the same direction. 

 In Bilharzia, where the sexes are separate, the male is so curled as to form a ' gynaecophoric ' canal 

 for the female. 



Self-impregnation occurs beyond a doubt in some instances. A single P. integerrimum has been 

 found in a Frog's bladder with sperm in the female passages. Von Linstow met with in Gammarus 

 Pulex a Distome (Z?. agamos) encysted, a single Distome in each cyst. Some individuals were sex- 

 less ; others had mature male organs ; others again mature female organs. Developing ova were 

 present in the oviduct. In this instance the male aperture is behind, the female in front of the 

 ventral sucker. Sometimes the structure of the apertures makes self-impregnation more than 

 probable. See Poirier, A. Z. Expt. (2), iii. 1885, pp. 582-85. Zaddach has actually observed the 

 act in D. cirrigerum (Z. A. iv. 1881, p. 427), encysted in Astacus. The ova of this animal are laid 

 in the cyst ; the parent dies ; the cyst decays, and the ova are then scattered. Leuckart states that 

 he has noticed sexual maturity attained in cases of prolonged encystation in a host, e. g. in 

 Ephemerae larvae (Parasiten, ed. 2, i. note, p. 98). For a discussion of the whole subject, see 

 Looss, Z. W. Z. xli. pp. 420-27. 



A canal has often been described as connecting the testis to the germ-duct, e g. by Zeller in P. 



