CHAPTER XII 



THE FLUKES 



General Account. — The flukes are animals of a very low order 

 of development in some respects and of very high specialization 

 in others. In shape they are flat and often leaflike, with the 

 mouth at the bottom of a sucker at the anterior end and with a 

 second little sucker, for adhesion, on the ventral side of the body. 

 They are all parasitic when adult and attach themselves to their 

 hosts, either externally or internally, by means of their suckers, 

 sometimes aided also by hooks. The development of the ner- 

 vous system is of a very low grade, and the only tendency towards 

 a brain is the presence of a small ganglion at the forward end of 

 the body which gives off a few longitudinal nerves. Sense organs 

 are almost lacking — there is usually no sense of sight and none 

 of sound; in fact no sensations whatever except a meager sense 

 of touch falls to the lot of these lowly animals. There is no blood 

 or blood system, the result being that the digestive tract and 

 excretory system are branched, often to a surprising extent, in 

 order to carry food to all parts of the body and to carry waste 

 products out from all parts. In these respects the flukes are 

 very primitive animals, but in other respects they equal or surpass 

 any other animals in their complexity. We would have to look 

 long to find more intricate and highly specialized reproductive 

 systems than they possess, and their life histories are so mar- 

 velously complex as to tax our credulity. We are accustomed 

 to think of a butterfly as having a wonderful life history in that 

 it passes through two phases of life, the first as a caterpillar, the 

 second as a mature butterfly, the two being separated by a third 

 inactive phase of existence. But by comparison with the flukes 

 this life history appears simple. Many flukes, especially those 

 which live as internal parasites in the land animals, pass through 

 four and sometimes even five distinct phases of existence, during 

 some of which they are free-living, and during others may para- 

 sitize successively two or even three different hosts. 



206 



