CHAP. XV Backboneless' Animals 229 



by budding forms temporary chains of eight or sixteen individuals 

 as if suggesting how a ringed worm might arise ; Gunda, with a hint 

 of internal segmentation ; and two parasitic genera — Graffilla and 

 Anoplodium — may be mentioned as representatives of this class. 

 You will find specimens by collecting the waterweeds from a pond 

 or seaweeds from a shore-pool, and the simplicity of some may be 

 demonstrated by observing that when they are cut in two each half 

 lives and grows. 



2nd Class. — Trematoda or Flukes. These are parasitic "worms," 

 living outside or inside other animals, often flat or leaf- like in 

 form, provided with adhesive and absorbing suckers. Those which 

 live as ectoparasites, e.g. on the skin of fishes, have usually a 

 simple history ; while those which are internal boarders have an 

 intricate life-cycle, requiring to pass from one host to another of a 

 different kind if their development is to be fulfilled. Thus the 

 liver-fluke (Distomum hepaticum), which causes the disease of liver- 

 rot in sheep, and sometimes destroys a million in one year in Britain 

 alone, has an eventful history. From the bile-ducts of the sheep 

 the embryos pass by the food-canal to the exterior. If they reach a 

 pool of water they develop, quit their egg-shells, and become for 

 a few hours free-swimming. They knock against many things, but 

 when they come in contact with a small water -snail (Lymrums 

 truncatulus) they fasten to it, bore their way in, and, losing their 

 locomotor cilia, encyst themselves. They grow and multiply in a 

 somewhat asexual way. Cells within the body of the encysted 

 embryo give rise to a second generation quite different in form. 

 The second generation similarly produces a third, and so on. 

 Finally, a generation of little tailed flukes arises ; these leave the 

 water-snail, leave the water too, settle on blades of grass, and lose 

 their tails. If they be eaten by a sheep they develop into adult 

 sexual flukes. Others have not less eventful life-cycles, but that of 

 the liver-fluke is most thoroughly known. If you dissect a frog 

 you are likely to find Polystomum integerrimum in the lungs or 

 bladder ; it begins as a parasite of the tadpole, and takes two or 

 three years to become mature in the frog. Quaint are the little 

 forms known as Diporpa which fasten on the gills of minnows, and 

 unite in pairs for life, forming double animals (Diplozoon) ; and 

 hardly less strange is Gyrodactylus, another parasite on freshwater 

 fishes, for three generations are often found together, one within 

 the other. The rjaost formidable fluke-parasite of man is Bilhania, 

 or Distomum hamatobium, common in Africa. 



3rd Class. Cestoda or Tapeworms. These are all internal 

 parasites, and, with the exception of one {Archigetes), which fulfils 

 its life in the little river -worm Tubifex, the adults always occur in 

 the food-canal of backboned animals. Like the flukes, they have 



