1 84 UNSEGMENTED "WORMS." 



Classification. The class Cestoda includes a number of families : - 

 Cestodariidse. No joints, one set of gonads. 



e.g. AriliigcteS) Caryopkyllatis, Ainphilina. 



Bothriocephalidte. Two weak flat suckers ; genital openings usually 

 on the flat surfaces. 



e.g. Bothriocephahis ; Ligula, with no suckers or joints but with 



serial gonads. 



Tetrarhynchidoe. With four protrusible proboscides armed with 

 hooks, parasites of fishes. 



e.g. Tetrarhynchiis. 

 Tetraphyllidse. With four very mobile suckers. 



e.g. Echeneibothriuni, Phyllobolhriuni. 



T:\miidce. With four suckers, often with apical hooks, with marginal 

 genital apertures. 

 e.g. Ttcnia. 



GENERAL NOTE ON PLATYHELMINTHES. 



The four classes, Turbellaria, Trematoda, Cestoda and Temnocepha- 

 loidea, constitute the Platyhelminthes or Flat-worms an interesting 

 group, because its members illustrate so well the progressive degenera- 

 tion associated with increasing parasitism, and also because of the 

 relatively great simplicity. The four classes are undoubtedly nearly 

 related, for forms like Temnocephala connect Turbellaria and Trematoda, 

 and the "monozoic" Cestodes like Archigetes, Amphilina, and Caryo- 

 phyllcTiiS) connect Trematoda and Cestoda. 



Among the most striking of the Platy helminth characters are the 

 nature of the excretory and reproductive organs and the condition of 

 the mesoblast. The excretory system, with its longitudinal trunks, its 

 ramifying canals, and "flame cells," is eminently characteristic, though 

 it occurs in more or less modified condition in higher forms. The repro- 

 ductive organs are complex, show division of labour, and are furnished 

 with ducts of their own, unconnected with the excretory system a con- 

 dition of affairs not common elsewhere. The presence of shells around 

 the eggs is another point of interest. It becomes of great importance to 

 the parasitic flukes and tape-worms, but occurs also in the free-living 

 Turbellaria. 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 in this connection, the two characters taken together 

 being held to indicate affinity with the Ctenophora. 



Class NEMERTEA. Nemertines. 



The ribbon-worms or Nemertines are interesting in many 

 ways, e.g. in being the simplest animals to have an open 

 gut, a closed blood-system, and, occasionally, haemoglobin ; 

 in having some very peculiar structures, notably a pro- 

 trusible proboscis and ciliated head slits ; in being in many 

 cases extraordinarily extensile and liable to break into pieces. 



