WOEMS. 121 



of those of Fishes ; the vulgar notion that the parasites of these 

 animals are dangerous to man has been shown to be entirely 

 erroneous. 



The Flukes infest animals of all kinds ; that which is most 

 dangerous to sheep, and the cause of much pecuniary loss {Distoma 

 hepaticum), is selected here as a type ; its structure is shown by a large 

 model (32), and its life-history by a series of diagrams (Figs. 10-13). 

 Here, again, we have a creature which infests two hosts. If the 

 larvse which escape from the sheep fall on wet 

 ground in or near a pool, they make their way -^'o- ^• 



to a small pond-snail {Limmca truncatida, Fig. 9), ^ 



into the lung-chamber of which they bore their 9 



way. On leaving them the larva may be, and Limnsea truncatula. 

 is, too frequently, eaten by a sheep, and makes 

 its way into the liver of that animal, where it causes the disease 

 known as the " liver rot." 



The damage done by the liver-fluke may be imagined from the 

 fact that in the winter of 1879-80 no less than three millions of 

 sheep died of rot in the United Kingdom ; this heavy loss is no 

 doubt largely due to the immense number of eggs to which a single 

 fluke may give rise. It has been estimated that every fluke may 

 produce, during its life, several thousands of eggs ; and in one case 

 Prof. A. P. Thomas found as many as 7,400,000 eggs in the gall- 

 bladder of a sheep which was suffering from rot, and which, at that 

 time, had in its liver about 200 flukes. 



The non-parasitic Flat-worms are shown, magnified, in the upper 

 parts of Cases I. & 11. The Turhellaria proper, without any or 

 with a simple or a branched intestine, but without a vent, are 

 represented by Gonvoluta and Thysanozoon : the general structure is 

 shown by a diagram in Case II., which is here reproduced (Fig. 14). 

 Planaria, Thysanozoon, and Bipalium serve to illustrate the forms 

 of members of this group. 



The Nemertine Worms {Nemertinea), with a straight intestine, 

 with a vent, and with a proboscis, may attain to a very considerable 

 length ; Lineus marinus, fov example, varies from 15 feet to 30 yards 

 in length ; GarinelJa and Lineus are represented by large figures, and 

 various species are shown in spirit (38-49). These forms, which used 

 to be very unsatisfactory to exhibit, on account of the great difficulty 

 of preserving them complete and uninjured, are now, with improved 

 methods, very satisfactorily shown, as the specimens purchased from 

 the Marine Biological Laboratory at Plymouth prove. 



