PHYLUM PLATYHELMINTHES 



201 



by some fluid derived from their host. Pigment occurs in some 

 of the ectoparasitic forms. 



The relationship of the Gestoda to the Trematoda is, as will be 

 subsequently shown, fairly close ; but though there are connecting 

 forms between the two classes, the shape of the average Cestode 

 is very different from that of such an average Trematode as the 

 Liver-Fluke. The body of an ordinary Cestode is of great length 

 sometimes extending even to a good many feet and relatively 

 narrow, being compressed into the form of a ribbon. One end, 

 which it will be convenient to designate anterior (though it may 



rb 



FIG. 203. Tetrarhynchus. n. nervous 

 system ; r. proboscides ; rs. sheaths, 

 with their muscles (>&.). (From 

 Leuckart, after Pintner.) 



FIG. 204. Tsenia echinococcus 

 (After Cobbold.) 



not, perhaps, correspond to the anterior end in a Trematode or a 

 Turbellarian), is, in most cases, attached to the host by means of 

 suckers and hooks placed on a rounded lobe, the head or scolex, 

 connected with the body by a narrow part or neck. The head is 

 usually rather radially than bilaterally symmetrical, with four 

 suckers and a circlet of hooks. The hooks, when present, are borne 

 on a longer or shorter retractile process, the rostellum, the long 

 axis of which is in line with the long axis of the body. In Bothrio- 

 cephalus and allied forms a pair of longitudinal grooves take the 

 place of suckers, and there are no hooks. In many Cestodes para- 



