PART II 

 THE INTERNAL PARASITES 



CHAPTER XIV 



PHYLUM II. PLATYHELMINTHES. THE FLUKES AND 



TAPEWORMS 



With but few exceptions all of the metazoan internal parasites come 

 into the old division Vermes, which brings together animals generally 

 worm-Hke, though widely differing in many respects. Compared thus 

 as a whole with animals usually rated below them in the zoological 

 scale, worms are readily distinguished in possessing differentiated 

 anterior and posterior extremities, the anterior directed toward their 

 forward movement and involving a head which contains a ganglionic 

 mass of nerve cells or, as it may be called, a rudimentary brain. Fur- 

 thermore, the body is bilaterally similar, and there is a dorsal and 

 ventral surface. The annulated worms, which include the higher 

 representatives, differ from the Arthropoda mainly in the absence of 

 articulated appendages to their body-segments, while the lack of a 

 notochord and gill-slits distinguishes them from certain lowly members 

 of the Chordata. Beyond these few points there is little to be said as 

 to the characteristics of the worms considered as a whole. 



The including in a single phylum of all invertebrates generally elongate 

 and without articulated appendages is systematically faulty in that it 

 brings together animals with structural differences of grand division 

 importance, though agreeing in an external form generally worm-like. 

 In most of the present-day systems of classification the worms are dis- 

 tributed into three, or at least two, phyla, the older class division 

 Platyhelminthes, or flat worms, being given grand division distinction. 

 Many authors also place the smooth-bodied Nemathelminthes and the 

 annulated worms in separate phyla, while another division, — the 

 Molluscoidea, has been created to dispose of the more or less related 

 moss-like Polyzoa (Bryozoa) and the mollusc-like Brachiopoda. An 

 objection to such arrangement is that groups poor in species, some of 

 them mainly of parasitic interest, are placed on the same basis as such 

 large and very important divisions as the chordates and arthropods, 

 thus giving them an undue prominence in a general consideration of the 

 animal kingdom. 



