238 



MANUAL OF ZOOLOGY. 



The Distoma (Fasciola) hepaticum (fig. 116, i) may be taken as the type 

 of the Trematoda. It is the common "Liver-fluke" of the sheep, and 

 inhabits the gall-bladder or biliary ducts, giving rise to the disease known 

 as the "rot." In form it is ovate, and flattened on its two sides, and it 

 presents two suctorial discs, the anterior of which is perforated by the 

 aperture of the mouth, whilst the posterior is impervious. Between the 

 suckers is the "genital pore," at which the efferent ducts of the reproduc- 

 tive organs open on the exterior. A branched water-vascular system is 



Fig. 116. Trematoda. i. Distoma hepaticum, the "Liver-fluke," showing the 

 branched alimentary canal. 2. Anterior extremity of Distoma lanceolatrwt. a An- 

 terior sucker ; b Posterior sucker ; c Generative pore ; d (Esophagus ; e Alimentary 

 canal. (After Owen.) 



present, and opens posteriorly by a small aperture. The alimentary canal 

 bifurcates shortly behind the mouth, the two divisions thus produced giv- 

 ing off numerous lateral diverticula, and terminating posteriorly in blind 

 extremities. The nervous system consists of two cephalic ganglia, giving 

 off filaments both forwards and backwards. The embryo of Distoma, on 

 its discharge from the egg, is inversely conical in shape, and is covered 

 with cilia ; but it appears soon to lose its cilia, and to become "cercarii- 

 form," abandoning its free aquatic life, and entering into the body of some 

 fresh-water mollusc. When its host is eaten by some mammal the larva 

 passes into its mature stage of development. The adult Distoma hepaticum 

 is found in the sheep, ox, horse, ass, hare, deer, &c., and occasionally in 

 man. 



In Distoma lanceolatum (fig. 116, 2) the intestine has not the ramose, 

 complex character of that of D. hepaticum. On the other hand, the ali- 

 mentary canal, after its bifurcation, is continued on each side of the body 

 to the posterior extremity without giving off any branches on the way, and 

 it terminates simply in blind extremities. It occurs in the liver of the ox, 

 sheep, pig, &c., and has been likewise detected in man. 



The only other Trematode which need be mentioned is the curious 

 Gyntzcophorus (Bilharzia) hcematobius, which occurs abundantly in the in- 

 terior of the blood-vessels of the human subject in certain regions (Egypt, 

 South Africa, Mauritius), and has also been found in a similar situation in 

 monkeys. The sexes are distinct in this form, the male being about half 

 an inch, whilst the female is nearly an inch in length, and both being ver- 

 miform in shape. 



ORDER TURBELLARIA. Leaf -like or vermiform Scolecids, 

 rarely parasitic, with a mouth and alimentary canal, and some- 

 times a body-cavity; integument ciliated. Sexes united or distinct. 



