226 PLATYHELMINTHBS. 



The ovary may be rounded (Fig. 185) or branched (Fig. 179); the 

 yolk-glands, which secrete vitelline matter, are much branched 

 tubular glands occupying the sides of the body (Fig. 179). The 

 oviduct, after receiving the ducts from the yolk-gland, is continued 

 as the convoluted, somewhat dilated, uterus which opens near the 

 male opening. The shell-gland is placed round the junction of the 

 oviduct and vitelline ducts, or round the first part of the uterus. 

 Here the egg is fertilised, and the vitelline particles come into 

 contact with the ova. In the Heterocotijlea (Polystomeae) the part 

 of the uterus into which the oviduct opens is dilated and called the 

 ootype. Here, or in the beginning of the uterus if there is no 

 ootype, each ovum acquires its investment of yolk, and is surrounded 

 by a strong shell ; it is then ready to be passed out through the 

 opening of the uterus. 



In a great many Trematodes, if not in all, there is a special paired 

 or unpaired canal, called the canal of Laurer,* which opens externally 

 in the dorsal middle line (Distomum), or laterally through two warts 

 on the sides of the body (Polystomum, Fig. 185), and internally into 

 the oviduct where it joins the yolk-ducts, or into the yolk-ducts near 

 this point. This canal, certainly in some cases, probably in all, is 

 functionally a vagina, and serves for the entrance of the spermatozoa re- 

 ceived in copulation, or otherwise, from another individual. In some 

 genera (Polystomum, Diplozoon, Octobothrium) there is a duct (vitello- 

 intestinal) connecting the oviduct with the intestine. It appears to 

 serve the purpose of carrying the superfluous yolk into the intestine. 



The so-called third vets defer ens, as a tube connecting the vas deferens with the 

 oviduct, appears not to exist. It is probable that the vitello-intestinal canal 

 (and possibly the canal of Lanrer) have been mistaken for a tube connecting the 

 oviduct with the male-duct in such a way as to permit of self-fertilization. 



The function of Laurer's canal is in some doubt. It has very generally been 

 interpreted as a vagina, but, excepting in Polystomum integcrrimum, in which 

 two worms have been detected in reciprocal copulation, with their male generative 

 openings applied to one of its openings in the other worm, the apposition of the 

 penis to it has never been observed. On the other hand, in the Distomic (dige- 

 netic) division reciprocal copulation with the penis of one worm inserted into 

 the uterine opening of the other has been observed in some forms, t and in one 

 case + the penis of one individual was found to be inserted into the adjacent 

 opening of its own uterus. These latter cases point to the view that the uterus 

 functions as vagina. Then the question arises, what is the function of the canal 

 of Laurer ? It has been suggested that its function is to carry off the superfluous 



* J. Fr. Laurer, Disquisitioncs anatomicae dc Amphistomo conico, 1830, 4. 

 t Holostomum scrpens, Monostomum faba, Distomum clavigerum, D. cylin- 

 draceum. 



+ Distomum cirrigerum. 



