60 LECTURE V. 



Distoma hepaticnm. Each half of the body has its own organs of 

 circulation, or secretion, and of generation. But for the anatomical 

 details of the Diplozoon, I must refer to Dr. Nordmann's elaborate 

 work. This truly paradoxical species, which is about two or three 

 lines in length, may be found attached to the gills of the bream. 



The genus Planaria, a fresh-water vermiform animal, and not an 

 internal parasite, is nevertheless referable, by its organisation, to the 

 order Trematoda. In the Planaria lactea the mouth is situated 

 upon a proboscis extending from the middle of the inferior surface of 

 the body. The alimentary canal almost immediately divides into 

 three principal branches, each of which is again subdivided into a 

 number of ramified cceca. The two posterior dendritic divisions are 

 obviously analogous to those in the Distoma hepaticum ; the anterior 

 division is azygos, and passes along the middle line to the anterior 

 extremity of the body. The generative organs of the Planarise are 

 androgynous, and are essentially the same as in the Distomata. The 

 ova of some species are attached by short filaments to the stems of 

 aquatic plants. The Planarice also propagate by spontaneous fission, 

 and are remarkable for the facility with which detached or mutilated 

 parts assume the form and functions of the perfect animal. 



The young of those species of the parasitic Trematoda, whose 

 development has been most satisfactorily observed, pass the first 

 period of their existence, like the PlanaricB, as free denizens of the 

 watery element. The Distomata tereticollis and cylindraceum are 

 however viviparous. The D. hepaticum and lanceolatum. are ovi- 

 parous. The egg-covering in these and many other species of Trema- 

 toda is provided with an operculum or lid. The young of the Dist. 

 hepaticum are spherical and ciliated like the polygastric infusoria ; and 

 they are provided at this active stage of their existence with a dark 

 coloured eye speck, and sometimes with a pair of ocelli. The young 

 of the Dist. globiporum are of an ash-grey colour, and without an 

 ocellus. All Trematoda appear first to present the ciliated infusorial 

 state before they attain their appointed place of abode, where they 

 undergo their final metamorphosis and develop their generative 

 organs. There can be little doubt that the sheep, which become in- 

 fested by the liver-fluke, swallow the ova or embryos that are ejected 

 upon the pastures or in the drinking places ; and the young flukes 

 must instinctively pass from the duodenum up the ductus choledochus 

 to the gall-bladder. Change of pasture, removal to uninfected 

 grounds, are, therefore, the first steps in the cure and prevention of 

 the rot-distemper caused by the Distoma. hepaticum. 



Of the order Acanthocephala, which includes the most noxious of 

 the internal parasites, fortunately no species is known to infest the 



