122 INVERTEBRATE ZOOLOGY 



the inturned scolex becomes everted and secures attachment to 

 the hning of the ahmentary canal of the new host. The scolex 

 is not affected by the digestive processes of the host but the cyst 

 appended to its free extremity disintegrates, leaving only the 

 scolex and neck which, through growth, produce the entire 

 strobila. 



The Scolex. — The scolex shows great diversity in form. In 

 members of the order Pseudophyllidea, the scolex is usually 

 provided with two longitudinal sucking grooves (Marsipometra, 

 Bothriocephalus, Abothrium, Ligula), though some instances 

 these are highly modified (Bothrimonus) and occasionally are 

 united to form an unpaired terminal adhesive organ (Cyatho- 

 cephalus). The scolex in members of this order is usually 

 unadorned except in Triaenophorus which bears conspicuous 

 chitinous hooks and in the primary scolex of Haplobothrium 

 which bears four proboscides, therein resembling the Trypa- 

 norhyncha. Members of the order Trypanorhyncha, which 

 occur chiefly in the digestive tract of marine fishes, have a 

 proboscis of varied form provided with four terminal, long, 

 cylindrical proboscides covered with minute spines (Tetrarhyn- 

 chus). In the order Tetraphyllidea are included cestodes with 

 a scolex bearing four cuplike bothria or suckers and with each 

 proglottid bearing numerous vitelline follicles (Proteocephalus, 

 Corallobothrium). The order Cyclophyllidea includes forms 

 having four cuplike or saucer-shaped suckers and usually a 

 terminal organ between the suckers termed a rostellum. In 

 this order, the vitellaria are usually posterior to the ovary and 

 occur in a single compact mass. This order of numerous families 

 includes most of the cestode parasites of the higher vertebrates 

 (Taenia, Anaplocephala, Hymenolepis, Dipylidium, Davainea, 

 and many other genera). 



Class Nemertinea 



The nemertines are usually included as a class under the 

 Plathelminthes because of their agreement with other fiatworms 

 in: (1) structure of the nervous system, (2) presence of a pro- 

 toncphridial excretory system, and (3) the strong development of 

 the mesoderm which renders the body highly parenchymatous. 

 On the other hand, they differ from other fiatworms in: (1) the 

 specialization of a vascular system so that distribution of nourish- 

 ment is not cared for by a combined gastrovascular system, (2) 



