CHAPTER XVIII. 



PLATYHELMIA TREMATODA. 



CLASS III. TREMATODA (RUDOLPHI). 



Order 1. Heterocotylea. 



Fam. 1. Monocotylidae. 

 ,, 2. Tristomidae. 

 ,, 3. Polystomidae. 

 4. Microcotylidae. 

 5. Gyrodactylidae. 



Order 2. Aspidocotylea. 

 Fam. Aspidobothridae. 



Order 3. Malacocotylea. 



Fam. 1. Amphistomidae. 



,, 2. Distomidae. 



3. Holostomidae. 

 4. Monostomidae. 

 5. Gasterostomidae. 

 6. Didymozoonidae. 



PARASITIC Platyhelmia which retain the mouth and alimentary 

 tract of the ancestor, but in which the epidermis not only loses its 

 cilia during embryogeny, but is apparently absent in the adult as a 

 distinct, continuous, cellular layer, having sunk into the mesoblastic 

 tissue, after secreting a thick, stratified, chitinous cuticle. Further, 

 in relation to their parasitic habits, suckers are developed at or 

 near the posterior end on the ventral surface, and also in the region 

 of the mouth. 



Historical. Our knowledge of Trematodes begins with Gabu- 

 cinus (1 547), who described the occurrence of the liver fluke in sheep, 

 which was, however, referred to by Jehan de Brie as early as 1379 ; 

 Leeuwenhoek (1695) added a form found in the herring; Swam- 

 merdam (1752) mentions a distome in the frog's lung; Roesel v. 

 Rosenhof (1758) gave a description and figure of a fluke (Poly- 

 stomum) which he discovered in the frog's bladder. Then came that 



