[260] The typical position of the genitalia is subject to many deviations, which are of importance in the differentiation of the genera and families. The following are some few of these deviations: (1) The genital pore remains on the ventral surface, but is situated beside or behind the ventral sucker, or it becomes marginal, and is then found in front of or beside the oral sucker, or at a lateral edge, or, finally, in the centre of the posterior border; the ducts also correspondingly alter their direction. (2) The ovary usually lies in front of the testes, not rarely, however, behind them or between them. (3) The three genital glands mostly lie together close in front of, or behind, the centre of the body; they may be moved far back, and may incidentally become separated one from the other. (4) The vitellarium may be single, in which case it then may lie in the central field. (5) A few forms possess but one, others several or numerous testes. Amongst the ectoparasitic trematodes there are also species with but one testis; but they mostly have several. As a rule, their uterus is short, but the oötype well developed. Special canals (vagina), single or double, are used for copulation, not the uterus. The vitelline ducts also communicate with the intestine through the canalis vitello-intestinalis (fig. 123). The spermatozoa do not differ essentially in their structure from those of other animals; the ovarian or egg cells are cells without integument and contain a large nucleus and a little protoplasm; the vitellaria also produce nucleated cells, in the plasm of which there are numerous yellow yolk granules; the yolk cells detach themselves, like the ovarian cells, from the ovarium, and pass into the oviduct to surround each ovarian cell in the oötype. They disintegrate sooner or later in the completely formed egg and are utilized as food by the developing embryo. DEVELOPMENT OF THE TREMATODES. (1) _Copulation._--Observation has demonstrated that the one or two vaginæ occurring in the ectoparasitic Trematodes are utilized as female organs of copulation, and that the copulation is cross; it is also known that Laurer’s canal, which was formerly generally regarded as the vagina, has only quite exceptionally, if at all, served the digenetic Trematodes as such--it appears to be homologous with the canalis vitello-intestinalis of the _Monogenea_[261]--but the terminal portion of the uterus, termed the metraterm, is used for copulation. Cross-copulation occurs as well as auto-copulation and auto-fecundation. The spermatozoa subsequently pass through the entire uterus, which is still quite short at the time the male organs are matured; the maturation of which, as usually is the case in hermaphrodites, precedes that of the female organs. It is only later with the onset of egg formation that the uterus is fully developed. Copulation, however, takes place also in the case of fully grown forms with completely developed uteri.