AFFINITY OF CESTODES TO TREMATODES. 105 



however, in which, among these Helminths, the free life of the larva 

 attains to greater biological independence than I have proved in 

 various ways to be the case among the Nematodes. Where we do 

 meet with a free larval form among them, its function is limited to 

 the search for and invasion of a suitable host (p. 61). Everywhere, 

 during this period of free life, nutrition and growth are in abeyance. 



It is evident, and has indeed been mentioned above, that on account 

 of this fact the proof of the relations to free-living animal forms is 

 made considerably more difficult. On account of an extensive adap- 

 tation to the conditions of parasitic life, the systematic characters of 

 the animals in question are considerably modified, and often rendered 

 wholly unrecognisable. 



Among the groups here mentioned there are two, the Cestodes and 

 Trematodes, which are very nearly related to each other, so nearly 

 indeed, that it is difficult to draw a clear distinction between them. 

 This announcement may seem startling, when merely the external 

 form of a Tcenia (Fig. 67) and of a Distomum (Fig. 68) is taken into 

 consideration, for at first sight there are scarcely two other Helminths 

 which differ so widely from each other in their external appearance. 



In one case, we find a ribbon-like body, perhaps some metres 

 in length, with head and segments; in the other, a body short, 

 simple, and flat ; in the one, suckers on the circumference of the 

 head, in the other, in the middle line of the anterior portion of 

 the body; in the former, an absence of mouth and of intestine, 

 in the latter, a well developed alimentary apparatus. Who, at 

 first sight, would expect to find resemblances among such oppos- 

 ing characters? But the question assumes another aspect, when 

 we recognise that what we call a tape- worm is not a single animal 

 like a caterpillar or millipede, but a whole colony, which furnishes 

 segments in regular succession, immediately behind the so-called 

 " head," which also represents a specialised individual the " Scolex " 

 (p. 37). Not the whole worm, but the single segment (Proglottis) 

 must be compared with the fluke ; and then we shall find, especially 

 in the structure of the sexual apparatus, which constitutes by 

 far the greatest portion of the whole internal organs, that there 

 are so many and such surprising similarities, that the close relation- 

 ship can no longer remain doubtful. Of course there are certain 

 differences between the two forms, especially in respect of the in- 

 testine and of the organs of attachment, but even these lose their 

 importance as soon as we extend our comparison over a large number 

 of species. 



In the first place, it has been shown that among the entopara- 

 sitic Trematodes there are a number of species which, like the 



