292 PLATHELM1NTHES. 



Order IV. Heteronemertini . 



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

 muscles; mouth behind brain; proboscis unarmed. Linens,* Micrura* and 

 Cerebratulus * (Meckelia) on our coast, with cerebral organs. Eupolia. 



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 TURBELLARIA are the most primitive; the Trematoda 

 and Cestoda ha^e descended from them. 



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

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

 mal pharynx and entodermal stomach, the latter many-branched 

 in the Polyclads, with three main branches in the Triclads, and 

 rod-like in the Rhabdocceles. 



4. Polyclads and Triclads are often united under the name 

 Dendroccela. 



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

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

 attachment to the host; several in the ectoparasitic forms; only 

 one or two suckers in the internal parasites. 



6. In the DistomicB there occur heterogony and alternation of 

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

 luscs, from the parthenogenetic eggs of .which develop cercarise 

 which become encysted Distomia3 in the second host, sexual Di- 

 stomiae in the third. 



7. Best known of the Distoma are D. liepaticum and D. 

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

 in the portal vein of man in warm climates. 



8. The CESTODA are characterized by the entire absence of 

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

 glottids. 



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

 Tided with suckers and frequently with hooks. It also produces 

 the proglottids by terminal budding. 



10. The proglottids contain an hermaphroditic sexual apparatus. 



11. The eggs produce a six-hooked embyro which must 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 fishes. 



