DR. THEODOK NEUMANN. 223 



cannot be absorbed must be ejected through the mouth 

 (distomum). However unheard-of such an arrangement 

 may be for all more highly developed animals, it is quite 

 a suitable thing where the quantity of stuff to be rejected 

 is extremely small. 



This alimentary canal may get shorter and shorter, so 

 that in some cases it consists only of two bulbs hanging 

 near the oesophagus (distomum heteroporum). This ar- 

 rangement is possible because the parasite does not by 

 any means depend on things which come to it through the 

 mouth only. Hand in hand with this reduction of the 

 alimentary canal goes a change in the animal's outer 

 cover which begins to take part in the process of feeding 

 the body. Far more than our own skin, which is permeable 

 to a great extent, the cover of the parasite allows the 

 liquids in the intestines of the host to permeate through 

 and be absorbed inside. Of course it must not be thick 

 and horny, as in animals which live in the open air, but 

 it must possess the faculty of passing liquid and gaseous 

 matters and enabling the parasite to combine nourish- 

 ment and breathing in one process. 



Such a way of providing food has certain effects on 

 the body of the parasite. The material which passes 

 through the skin reaches at first those parts of the body 

 closely behind it; only after they are fully provided for, 

 can the others, which lie still deeper, receive their share. 

 In consequence of this arrangement, and in order to 

 secure an equal and abundant nutrition of the body, it 

 is desirable to bring all its parts as near the surface as 

 possible, or, what is the same thing, to make the latter 

 as large as possible. Consequently these animals have a 

 characteristically flat, leaf-like shape, for only thus is it 

 possible to have the whole surface of the body take i)art 

 in the general nutrition of the individual. 



Thus, when the whole surface takes part in the absorp- 

 tion of food, it is easy to believe that in a great many 



161 



