222 THE ANIMAL PARASITES OF MAN 



copulatory organs are formed by hooks projecting into the lumen 

 of the terminal portion of the vas. 



The GENITAL PORE, which is the opening from the genital sinus on 

 to the surface, is generally situated at or near to the median line on 

 the ventral surface and in the anterior region of the body ; in most 

 of the Distomata it is in front of the ventral sucker, in other cases, 

 e.g., in the Cryptocotylince, it is behind. 1 



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 ootype. They disintegrate sooner 

 or later in the completely formed egg and are utilized as food by the 

 developing embryo. 



DEVELOPMENT OF THE TREMATODES. 



(i) Copulation. Observation has demonstrated that the one or 

 two vaginae 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 2 but the terminal 

 portion of the uterus, termed the metraterm, is used for copulation. 

 Cross-copulation occurs as well as auto-copulation and auto-fecunda- 

 tion. 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 



1 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 : (i) 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 ootype well developed. Special canals (vagina), single or double, are used for copula- 

 tion, not the uterus. The vitelline ducts also communicate with the intestine through the canalis 

 vitello-intestinalis (fig. 123). 



2 Monogenea : Trematoda in which the anterior sucker, if present, is double. Development 

 without an intermediate host. 



