TREMATODA. 



'59 



causes the serious disease called liver rot. The animal is 

 flat, oval, and leaf-like, measures almost an inch in length 

 by half an inch across the „, 



broadest part, varies from „ / 



reddish brown to greyish 

 yellow in colour. As the 

 word Dtstomum suggests, 

 there are two suckers, — 

 an anterior, perforated 

 by the mouth ; a second, 

 imperforate, a little 

 further back on the mid- 

 ventral line. 



There is a muscular 

 pharynx and a blind 

 alimentary canal which 

 sends branches through- 

 out the body. The 

 nervous system consists 

 of a ganglionated collar 

 round the pharynx, from 

 which nerves go forward 

 and backward ; of those 

 which run backward, 

 the two lateral are most 

 important. Although the 

 larva has eye spots to 

 start with, there are no 

 sense organs in the adult. 

 The body cavity is repre- 

 sented by a few small 

 spaces. Into these there 

 open the internal ciliated 

 ends of much-branched 

 excretory tubes, which 

 unite posteriorly in a 

 terminal vesicle opening 

 to the exterior. 



Fig. 70. — Structure of liver fluke. — After 

 Sommer. From ventral surface. The 

 branched gut [g. ) and the lateral 

 nerve (l.n.) are shown to the left, the 

 brandies of the excretory vessel (e.v.) 

 to the right. 



m., Mouth; ph., pharynx; g: t lateral head 

 ganglion ; v.s., ventral sucker ; c.s., position 

 of cirrus sac ; an arrow indicates the ex- 

 cretory aperture. 



The reproductive system is hermaphrodite and complex. From 

 much-branched testes, spermatozoa pass by a pair of ducts (vasa 

 deferentia) into a seminal vesicle lying in front of the ventral sucker. 



