WORMS AND CCELENTERATA 275 



large ovaries, which greatly complicate the sexual 

 apparatus. 



In fact, we very often find both sexes united in one 

 parasite, and we then call the animal an hermaphrodite. 

 This is not difficult to understand. The parasite is 

 often alone in its host, as is generally the case with 

 the tape-worm ; and it has then to develop both 

 spermatozoa and ova, or remain barren. Sometimes, 

 it is true, there may be two or more tape-worms in one 

 host, and this is more frequent in the case of other 

 parasites. Then mutual fertilisation is possible. 

 Hermaphrodism has been introduced amongst the 

 parasites to prevent the animals that live in isolation 

 from perishing without doing something towards the 

 maintenance of the species. 



We have now examined the organisation and habits 

 of parasites from our point of view, but there is one 

 point we have not yet considered the change of hosts. 

 This plays a very important part in parasites, and we 

 have already referred to several that pass their early 

 stage in one animal and come to sexual maturity in 

 another. An attempt has been made to explain this 

 remarkable phenomenon by regarding the first host 

 as the original one, in which the parasite became 

 sexually mature in former ages. After the rise of the 

 vertebrates it adapted itself to these, so that the 

 parasites taken with the flesh of the first host into the 

 alimentary canal of the second did not die, but only 

 reached sexual maturity there, as the conditions are 

 the best conceivable for parasites in the vertebrate. A 

 good deal can be said in support of this view, especially 



