292 PL A TIIELMINTHES. 



Order IV. Heteronemertini. 



Body wall with several muscular layers, the nervous system in the 

 muscles; mouth behind brain; proboscis unarmed. Liiieus* Mieru7-a * a.nd 

 Cei-ebratulus *■ (3JeckeUa) on our coast, with cerebral organs. EujMlia. 



Summary of Important Facts. 



1. The PLATHELMINTHES are bilateral animals of flattened 

 form whose nervous system consists of a supracesophageal ganglion 

 and lateral nerve trunks; the excretory system of branched water- 

 vascular tubes (protonephridia). 



2. The TuEBELLARiA are the most primitive; the Trematoda 

 and Cestoda have descended from them. 



3. The Turbellaria are ciliated externally. Tliey have no anus 

 and no circulatory system. The digestive tract consists of ectoder- 

 mal p)harynx and entodermal stomach, the latter many-branched 

 in the Polydads, with three main branches in the TrickuU, and 

 rod-like in the Ilhahdncmles. 



4. Polyclads and Triclads are often united under the name 

 Dendroccela. 



5. In the parasitic Teematoda the cilia are entirely lost or 

 confined to the larval stages. Hooks and suckers are present for 

 attachment to the host; several in the ectoj^arasitic forms; only 

 one or two stickers in the internal parasites. 



0. In the Distomice tliere occur heterogon}' and alternation of 

 hosts. From the egg arises a sjiorocyst, always parasitic in mol- 

 luscs, from the parthenogenetic eggs of which develoji cercari* 

 which become encysted Distomia? in the second host, sexual Di- 

 stomifB in the third. 



7. Best known of the Bisfoma are I>. licpaticum and D. 

 lanceolatum (rare in man, common in sheep) and D. licematojiium 

 in the portal vein of man in warm climates. 



8. The Cestoda are characterized by tlie entire absence of 

 digestive tract, and usually by the existence of scolex and pro- 

 glottids. 



'.). The scolex is the organ of attachment, and as such is pro- 

 vided with suckers and frequently with hooks. It also produces 

 the proglottids by termiiuil l)udding. 



10. The proglottids contain an hermaphroditic sexual apparatus- 



11. The eggs produce a six-honked embvro which mtist pass 

 into an intermediate host. This is accomplished either by taking 

 the eggs in passively with the food, or the embryo must pass into 

 the water, where it infects iishes. 



