PHYLUM IV, PLATYHELMINTHES (FLAT-WORMS) 



The old phylum Vermes (Latin vermis, a worm) was a com- 

 prehensive group of no exact classificatory value, including 

 soft, invertebrate animals, many bearing a general appearance 

 to the common earthworm. This is divided into four phyla, — 

 Platyhelminthes (flat-worms), Nemathelminthes (round- w^orms) , 

 Trochelminthes (wheel-worms) and Annulata (ring-worms). 



Body unsegmented, usually flattened from above downward. 

 Digestive canal without an anus, the w^aste being carried off by 

 the excretory system. No blood vascular system is present. 

 The sexes are united in the same individual. Many forms are 

 parasitic. This is the lowest phylum to assume a pronounced 

 bilateral symmetry. The Nemertinea differ from the other 

 orders of this phylum as described above in the possession of an 

 anus, a blood vascular system and a protrusible oesophagus. 

 The nemertines are covered by a very slimy secretion. Fossil 

 flat-worms are extremely rare; they are known from the 

 Pennsylvanian to the present day. 



Derivation of name. — Greek platys, flat, + helmins, worm, 

 referring to the flattened appearance of the body. 



Living examples are the Planaria (fresh water flat- worm), 

 Distomum hepaticum (liver-fluke, parasitic in sheep), and 

 Tcenia solium (common tape-worm, parasitic in man and the 



pig)- 



PHYLUM V, NEMATHELMINTHES (THREAD-WORMS) 



Body unsegmented, long and cylindrical, with mouth and 

 anus at opposite ends. Sexes usually separate. Mostly para- 

 sitic. Fossils extremely rare. A few examples of this and the 

 preceding phylum have been found fossil as parasites upon insects 

 from the upper Paleozoic coal measures and the Tertiary amber. 



140 



