36 THE LIVER-FLUKE. 



present. There is no alternation of generations, and tliey are 

 ecto-parasitic — that is, they live outside or upon their host in- 

 stead of internally, as is the case in the Distomata. 



Struatiire of a Trematode. — A typical fluke, such as the 

 Sheep-fluke {Distomum liepaticum), has a bifurcated alimentary 

 canal with simple or ramifying branches, and terminates in a 

 cul cle sac; there is thus no anus. The excretory apparatus 

 consists of a network of fine tuljes which converge into canals ; 

 these become gradually larger, and end in one or several longi- 

 tudinal vessels, which finally culmmate in a dilated pulsating 

 vesicle that opens to the outside by a pore, the caudal fora- 

 men. The flukes are hermaphrodites. The male organs are two 

 tubular testes, that unite into a " cirrus " or penis, which is 

 surrounded by a sheath. The female organs consist of an ovary 

 and oviducts. Attached to the ovary are two glands known as 

 the " albummous glands,'' which secrete a granular fluid. There 

 is also a shell-gland, uterus, and vagina, which open by the vulva 

 close to the male organ. The ova of the fluke are found in the 

 uterus, where they undergo the first stage of development — 

 namely, segmentation. A nervous system is developed, consist- 

 ing of two sub-CBSophageal ganglia united by a transverse com- 

 missure, and a single sub-oesophageal ganglion joined to the 

 others by two lateral commissures and a few nerve-filaments. 



Trematodes are found in the intestine, biliary canals, bladder, 

 respiratory apparatus, and in the sub-orbital sinus of birds. One 

 of the most important is the Liver-fluke {Didomum liepaticum), 

 which produces the " liver-rot " in sheep or the so-called disto- 

 ■matosis. Another species, also found in the liver region of the 

 sheep, is D. laneeolahim, a smaller and more p)ointed species. 



The Liver-fluke {D. hepatieum) (fig. 7).— The body of this 

 destructive fluke is leaf-like, flattened, and of an irregular pale- 

 brown colour. It is about an inch in length and half an inch 

 across at the widest part ; it is oval and lanceolate in shape 

 broader and rounder in front, where it suddenly contracts to 

 form a kind of neck. The cuticle is studded A^^ith numerous 



