TllEMATODA. 4 1 



Trematode worms go through a kind of alternation of genera- 

 tions, and live in two distinct hosts during their changes. 



"We can distinguish two groups of Flukes — (1) the Distomata, 

 which have two suckers and no hooks, and present an alter- 

 nation of sexual and asexual generations, — the asexual phase 

 always taking place in some mollusc. (2) The Polystomata,^ 

 which have two small lateral anterior suckers and one or two 

 posterior suckers. Hooks, usually two in numher, are also 

 present. There is no alternation of generations, they live out- 

 side or upon their host (ecto-parasites) instead of internally 

 (endo-parasitica or entozoa), as is the case in the Distomata. 



Structure 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 

 aul de sac, there is thus no anus. The excretory apparatus 

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

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

 tudinal vessels, which finally culminate in a dilated pulsating 

 vesicle that opens to the outside by a pore, the caudal foramen. 

 The Distomum 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 " albuminous 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 develop- 

 ment — namely segmentation. Another important species, Bil- 

 harzia hcernatohia, has a male and female form. A nervous 

 system is developed, consisting of two sub-cesophageal ganglia 

 united by a transverse commissure, and a single sub-oesophageal 

 ganglion joined to the others by two lateral commissures, and a 

 few nerve-filaments. 



^ These are now spoken of as (i) Digenea, and (ii) Mo-nogenea. 

 ^ This is often called Fasoiola hepatica. 



