WORMS AND CCELENTERATA 273 



water-animals, and are therefore the most widespread 

 of all parasites. In many of the tracheates parasitism 

 has gone so far that their respiratory organs have been 

 affected. Thus the pentastomum we referred to above 

 has lost its tracheae, and breathes through the skin, 

 like the worms. 



We have now seen that the nature of parasitism 

 entails a simplification of many organs, but other organs 

 are all the more elaborately developed in parasites. 

 These are the sexual organs. In the first place, it 

 is very easy for the parasite to supply them with plenty 

 of food. It is devoid of so many organs that require 

 their proportion of the food in other animals. Hence 

 we find, as a matter of fact, the sexual organs of 

 parasites swell in proportion to the thoroughness of 

 their parasitism. When we study the anatomy of a 

 distomum we have some difficulty in finding the other 

 organs on account of the pronounced sex organs. In 

 the tape-worm the other organs occupy an evanescent 

 space in comparison with the testicles, the ovary, and 

 their glandular appendages. 



It is not merely possible, but necessary, for the 

 parasites to have this preponderant development of the 

 sexual organs. It is just as difficult for the parasite to 

 maintain its species as it is easy to support itself. 

 When the host, especially of internal or fixed para- 

 sites, perishes, the parasites must die also, as a rule. 

 Hence its ova must always be conveyed to new animals. 

 In the case of the sacculina this is comparatively simple, 

 as we saw in the last chapter. From the egg is 

 developed an active larva, which seeks a new host. But 



