THE NEW EVOLUTION 



which intergrade between the trematodes and the tapeworms. 

 As the spiny-headed worms are now considered to be more or less 

 closely related to the tapeworms, it is evident that the union 

 of the tapeworms, spiny-headed worms, flukes and turbellarians 

 in a single phylum is a logical disposition of these groups. The 

 nematodes or thread-worms, however, seem at first sight to have 

 very little in common with any one of these groups. They have 

 been generally associated with the spiny-headed worms and the 

 gordian worms in the phylum Nemathelminthes. The nervous 

 system of the nematodes consists of a ring about the gullet from 

 which six anterior and six posterior trunks arise. There are also 

 other suggestions of radial symmetry. These traces of radial 

 symmetry are the only features which definitely align the nema- 

 todes with anything else. The nematodes entirely lack ciliated 

 tissue, all of them molt, at least in the young stages, most of 

 them possess a spinneret, many are more or less sharply and dis- 

 tinctly segmented externally, and some have segmented append- 

 ages as in the arthropods and some rotifers. But the differences 

 between the nematodes, the arthropods and the rotifers are much 

 greater than the resemblances. The characteristic nematode 

 features seem in a most extraordinary way to supplement the 

 features found in the tapeworms, flukes, etc., thus indicating that 

 the Vermiformes as here understood is really a natural group. 

 The gordian worms or "hair-snakes" which have essentially 

 the body form of nematodes but are otherwise very different, are 

 radially symmetrical at the anterior end in the larval stage. We 

 may regard the tapeworms, spiny-headed worms, flukes, turbel- 

 larians, nematodes and gordian worms as wholly anomalous and 

 without any affinities to each other or to anything else — each 

 as a sort of zoological accident — or we may regard the occurrence 

 in all of them of traces of radial symmetry as significant and as 

 showing that they differ equally from the radially symmetrical 

 animals on the one hand and from the bilateral animals on the 

 other. We cannot adopt the former alternative without imply- 

 ing a lack of order in the animal world, which is an inconceivable 

 assumption. The only reasonable course is to accept the latter 

 alternative and to consider all these creatures collectively as 

 representing a rather heterogeneous phylum intermediate between 

 the radially symmetrical coelenterates and the more complex 

 bilaterally symmetrical animals. 



[2-74] 



