196 



UNSEGMENTED "WORMS." 



Tetraphyllidse. With four very mobile suckers. 



e.g. Echeneibothrium , Phyllobothriuui. 



Tseniidre. With four suckers, often with apical hooks, with marginal 

 genital apertures. 



e.g. T&nia. 



GENERAL NOTE ON PLATY- 



HELMINTHES 



The four classes Turbellaria, 

 Trematoda, Cestoda, and Temno- 

 cephaloidea, constitute the Platy- 

 helminthes or Flat-worms an 

 interesting group, because its mem- 

 bers illustrate so well the progressive 

 degeneration associated with increas- 

 ing parasitism, and also because of 

 the relatively great simplicity. The 

 four classes are nearly related, for 

 forms like Temnocephala connect 

 Turbellaria and Trematoda, and the 

 "monozoic" Cestodes like Archi- 

 getes, Amphilina> Caryophyllaus, 

 and Gyrocotyle connect Trematoda 

 and Cestoda. It is probable that 

 both Cestodes and Trematodes arose 

 from a Turbellarian stock. 



Among the most striking of the 

 Platyhelminth characters are the 

 nature of the excretory and repro- 

 ductive organs and the condition of 

 the mesoderm. The excretory system, 

 with its longitudinal trunks, its 

 ramifying canals, and "flame-cells," 

 is characteristic. The reproductive 

 organs are complex, show division 

 of labour, and are furnished with 

 ducts of their own, unconnected with 

 I. The ordinary Cysticercus type the excretory system a condition 



with one head (H.). not common elsewhere. The pres- 



II. The Coenurus type, with many ence of shells around the eggs is 



^^ Ac . another point of interest. It be- 



producing many parasitic flukes and tape- worms, but 

 occurs also in the free - living 

 Turbellaria. The formation of yolk 

 cells from a specialised part of the ovary (yolk gland) is also note- 

 worthy. There is no true body-cavity, the space between gut and 

 body-wall being filled with a packing tissue ; the absence of an anus 

 is also important, the two characters taken together being held to 

 indicate affinity with the Ctenophora. 



HI 



FIG. 102. Diagrams of 

 bladder-worms. 



heads. 



T X t 

 capsules 

 heads. 



