WORMS AND CCELENTERATA 283 



shaped creatures develop from them ; these swim 

 about, and bore into a water-snail, in the interior of 

 which they grow into tube -like bodies. Inside the 

 tubes are a quantity of small, tadpole-shaped creatures. 

 These abandon the tube and the host, and seek a 

 new victim, inside which they incapsulate themselves ; 

 though they may do this, without migrating, on 

 aquatic plants. If the sheep eat these plants, the 

 distoma develop from the capsules in their intestines, 

 and pass on to the liver. The other species, which 

 have incapsulated in a second host, reach sexual 

 maturity when the new host is eaten. 



The adaptations of the distoma are enormously 

 varied, and may be very complicated. Especially 

 interesting is the Distomum macrostomum, which is 

 found in birds. When the eggs of this animal reach 

 the plants that grow on the edge of brooks and ponds, 

 they are swallowed by the snails, which gnaw the 

 plants. In these they develop into curious tubes that 

 surround the intestines of the snail, and also run out 

 into its feelers, which they distend and distort into thick 

 tubes. Inside the tubes, especially those in the antennae, 

 the young distoma are developed, and the tubes contain- 

 ing them now become coloured with green and white 

 bands, and move about. The movement increases, the 

 snails' antennae burst, and the tubes fall to the ground, 

 where they creep about and look very much like insect- 

 larvae, or caterpillars. The birds really take them to be 

 edible animals of that kind, swallow them, and so take 

 the brood of distoma into their bodies, where they reach 

 sexual maturity. Here we have a most peculiar case of 



