I IO 



SYSTEMATIC ZOOLOGY 



the mid-ventral line is the genital opening ; there may be a single 

 opening for the two sexes or a separate one for each, lying side 

 by side. A short distance posterior to the genital pore is the 

 ventral sucker, and at the extreme posterior end of the body 

 lies the opening to the excretory system. The mouth leads into 

 a muscular pharynx succeeded by a short oesophagus, which 



connects with the U- 

 shaped intestine, in 

 some cases with 

 numerous lateral 

 branches and with no 

 anal opening. The 

 excretory system con- 

 sists of branched 

 tubules opening pos- 

 teriori}-, and the 

 nervous system re- 

 sembles that in the 

 Turbellaria. The re- 

 productive organs are 

 very highly developed 



and self-fertilization 

 seems to be the rule. 

 The development 

 of the Trematoda is 

 somewhat varied. In 

 most of the ectopara- 



FlG. ioo bis. Distomum hepaticum, showing the anatomy; 



the ventral sucker appears as a white disc, i, the dark 



tubes represent the excretory system opening by a pore 



at the posterior end of the body; the lighter tubes form 



the digestive tract; 2, the white dots are the lestes, which 



connect wiih two long tubes; the branching tubes in the 



center constitute the female organs , the nervous system is 



represented by ganglia in the head region and two fine 



cords passing backward. (Photographed from a Leuck- 



art-Nitsche wall-chart, by the author; permission of Dr. sites it is verv simple. 



C. Chun.) , . 



— the young when 

 hatched from the c%g resemble closely the adult. In most of 

 the cntoparasites development takes place in one of two ways: 

 in the one case a ciliated larva is produced which passes into a 

 fit st host and does not become sexually mature until it reaches 

 its second host; in the other case, of which Distomum furnishes 

 an example, the change is more complicated. The fertilized 

 eggs of the fluke pass out through the alimentary canal and. on 

 coming in contact with the water, the larva- which have devel- 

 oped within them escape and swim about until thev have met a 

 certain species of snail, into whose both- they bore their way : 



