170 UNSEGMENTED " WORMS? 



and these in favouring circumstances grow into the adult 

 flukes. 



The above history has been independently worked out by Leuckart 

 and Thomas. 



It will be noted that the sporocyst is the modified embryo, but that it 

 has the power of giving rise asexually to redke. These develop, how- 

 ever, from special cells of the sporocyst which we may compare to 

 precociously developed parthenogenetic ova. Though the reproduction 

 is asexual, it is not comparable to budding or division. The same 

 power is possessed by the redise, and there are thus several (at least two) 

 asexual generations between the embryo and the adult. Finally, it 

 must be clearly understood that the cercaria is the young fluke. 



The disease of liver rot in sheep is common and disastrous. It has 

 been known to destroy a million sheep in one year in Britain alone, and 

 in the winter, 1879-80, the mortality attributed to fluke disease was 

 estimated at three millions. It is especially common after wet seasons, 

 and in damp districts. The preventives suggested are drainage of 

 pastures and dressings of lime and salt ; destruction of the eggs, the 

 snails, infected manure, and diseased sheep. It is usual to give the 

 infected sheep some salt and a little dry food. 



Classification. 



Trematodes with direct development Monogenetic. 



e.g., Polystoimim integerrimum. This form with many suckers 

 is often found in the bladder of the frog. It attaches itself 

 in its youth to the gills of tadpoles, passes thence through 

 the food canal to the bladder, where it develops slowly 

 for years. 



Gyrodactylus, found on the gills and fins of fresh-water fishes. 

 It is viviparous, but the embryo, before it is extruded, 

 itself contains an embryo, and this in turn another, so 

 that three generations of embryos are represented sim- 

 ultaneously. 



Diplozoon paradoxum, consists of two individuals united. 



The single embryo (Diporpa) is at first free swimming, 



but becomes a parasite on the gills of a minnow, and there 



two individuals unite very closely and permanently. 



Tristomtim, with three suckers, is not uncommon on the 



skin of some marine fishes. 



Trematodes with indirect development Digenetic. 

 e.g., Fasciola or Distonia. 



Bilharzia, or Gyncecophorus h&matobius, a dangerous parasite 

 of man, widely distributed in Africa. It infests the urinary 

 and visceral blood vessels. The sexes are separate, and 

 the male carries the female inserted in a groove. 

 Monostomum, a form with one sucker. 



The relationships of the class are on the one hand with the free living 

 Turbellarians, on the other hand with the parasitic Cestodes. 



