238 



PLATYHELMINTHES. 



as ciliated larvae (of the miracidium type), which probably make their way into 

 an intermediate host (fish, amphibian, mollusc, mammal), in which they develop 

 into a larva known as Tetracotyle (usually with four suckers). The Tetracotyle 

 probably becomes directly transformed into the adult when the intermediate host 

 is eaten by the permanent host. This kind of development, in which there is 



a change of host but no alternation of genera- 



Dra Ms tions, is called metastatic. Diplostomum 



Brds. ; Polycotyle Will, and Schm. ; Hemi- 

 stomum Brds. (Fig. 189) ; Holostomum 

 Nitzsch. 



Fam. 2. Amphistomatidae. Digenetic 

 forms with two suckers ; the posterior (ven- 

 tral) sucker is terminal, and on it or just in 

 front of it there are sometimes numerous 

 papillae or pits for attachment. Genital 

 opening median, ventral, in anterior third of 

 body. Eggs with operculum, without fila- 

 ment. In alimentary canal of Vertebrata. 

 Amphistommn Rud.; Diplodiscus'Die.s.', Gas- 

 trodiscus Cobb.; Homalogaster Poir.; Gastro- 

 thylax Poir. ; Aspidocotyle Dies. 



Fam. 3. Distomatidae. Digenetic forms 

 with two suckers ; the posterior sucker is on 

 ventral surface. Genital opening median, in 

 anterior third of body, anterior to ventral 

 sucker, rarely behind the latter or lateral. 

 Eggs usually with operculum, without fila- 

 ment. Laurer's canal usually present. In 

 alimentary canal and its appendages of Verte- 

 brates. Distomum Retz., D. hepaticum L. 

 liver-fluke (Fig. 179) ; with conical anterior 

 end and numerous spine-like prominences on 

 the surface of the broad, leaf-shaped body, 

 which is about 33 mm. long. In the bile- 

 ducts of sheep and of other domestic animals, 

 causing sheep-rot. It is occasionally found 

 in man, and bores its way into the portal 

 vein and into the system of the vena cava. 

 The embryo is hatched after the egg has been 

 some time in water as a ciliated larva (Fig. 

 182), with an x-shaped eye-spot ; this passes 

 into the water-snail Limnaea truncatula, and 

 there casts its ciliated skin and emerges 

 as a sporocyst. The sporocyst produces 

 rcdiae, which produce more rediae or Cer- 



cariae. The Cercariae, which are provided with long tails, leave the host, 

 swim about for a short time in water, and then encyst upon foreign objects, 

 e.g., blades of grass. In this condition they are eaten by the sheep. 

 D. crassum Busk, perhaps identical with D. rathouisi Poir. (Fig. 190) in the 

 intestine of Chinese. D. lanceolatum Mehlis, body lancet-shaped, 8- 9 mm. long, 

 lives in same place as D. hepaticum; the miracidium is pear-shaped, ciliated on 



FIG. 189. Hemistotmim clathratum 

 Dies., from the gut of Lutra bra- 

 siliensis, ventral view (from Bronn, 

 after Brandes). M.s oral sucker; 

 K.st. ovary ; Ut. uterus, the open- 

 ing of the uterus is rather indis- 

 tinctly rendered, just below the 

 lower Ut. ; H testis ; V.s. vesicula 

 seminalis ; Dr.a openings of glands 

 Dr. ; z ventral adhesive apparatus, 

 the ventral sucker is just in front 

 of this ; Sch shell-glands. 



