WORMS AND CCELENTERATA 269 



and surround themselves with a capsule. If the larva 

 is eaten by a larger insect, the capsule bursts in its 

 stomach, the worm becomes free, and develops to the 

 adult stage in its new host ; it abandons this home in 

 wet weather, and returns to the water to lay its eggs. 



The life-story of this Gordius aquaticus brings us to 

 one of the most interesting phenomena of the animal 

 world, parasitism. There are many parasites among its 

 relatives, the nematodes, which form the lower class of 

 the round worms ; but there are still more amongst the 

 platodes. Generally speaking, the vermalian stem is 

 the richest in parasites in the whole animal world, the 

 articulates alone approaching them in this respect. In 

 comparison with the parasites of these two stems, the 

 rest are insignificant ; the protozoa alone of other groups 

 provide a large number, including the notorious malaria 

 parasites. In the plant world again we have the bacteria 

 and other fungi. 



Rudolf Leuckart, the chief authority on the subject, 

 defines parasites as creatures that find food and shelter 

 in a living organism. According to this definition there 

 are, of course, parasites amongst the plants, some with 

 other plants, some with animals, as hosts. In our view 

 of the origin of living things all parasites must have 

 descended from free-living organisms. 



This statement can easily be proved in the case of 

 animal parasites. We have already seen something of 

 parasitic crustaceans. We saw that they pass their 

 youthful stages as free organisms, and can hardly be 

 distinguished from those of other crustaceans. We may 

 now add that we find a large number of transitional 



