1^58 rLATHELAIIXTIIES 



the worm thus produced escapes from tlie rest of the pihdium, which 

 iiuickiy dies. 



Order I. Protonemertini. 



Xcrvous system outside the muscles; no st\lels in the proboscis; moulli 

 behind lirain. Cjriiullj.''' 



Order II. Mesonemertini. 



Xervous system in tlie muscles; inoulli behind brain; no stylets. Crphalolliri.x* 



Order III. Metanemertini. 



Xcryous system inside the muscles, mouth in front of brain; proboscis as a 

 rule widi slylets. iit'ojniiuiii's'-' and some species of riiraslniiiiiii,'''' terrestrial. 

 .1 ;»/>/( //lorHi* (fig. 230, fresh yater), XcrloiiriiifrlfS.'-' Muhn'obdclla/''- leech- 

 like, with posterior sucker, jiarasiuc in lamellibranchs. 



Order IV. Heteronemertini. 



Several muscular layers; neryous system in the muscles; mouth behind 

 brain; proboscis unarmed. Liittinr' Muriird/-' Cirrbraliilus* Zyi;^^upoli^J.'^ 



Summary of Important Facts. 



1. The TLATHELMIXTHES are daltened bilateral animals with- 

 out cielom, whose neryous system consists of a supraivsophageal ganglion 

 and lateral nerye trunks; the excretory syslenr of branched protonephridia. 



2. The TuRBELLARi.v are the most primitiee; the Trematoda and 

 Cestoda haye descended from them. 



^ The Turbellaria are ciliated externally. They haye no anus and 

 no circulatory systeni. The tligesti\e tract consists of ectodermal 

 pharynx and entodermal stomaeli, tire latter man\'-branched in tire 

 Folyi'laJs, with three main branches in tire Triiiads, and rod-like in the 

 JUialniorirlrs. 



4. Polycdads and Triclads arc often united under the name Dcmiro- 

 caia. 



5. In the parasitic TRr:ii.\TOP.v the cilia are entirely lost or conlhied 

 to the larxal stages. Hooks and suckers are preserit for attachment lo the 

 host; several in the ectoparasitic forms; only oire or two suckers in the 

 intenral parasites. 



6. In the Dislomur there occur heterogony aird alternation of hosts. 

 From the egg arises a sporocyst, always parasitic in molluscs, fronr the 

 partheirogenetic eggs of which dewdop cercaria' which become encysted 

 Distom.i;c in the second host, sexual Distomia' in the tliird. 



7. Best known of the Distoma are D. Iir/^alirii))! and D. huucoUUitm 

 (rare in man, common itr sheep) and D. lucinatobium in the portal vein of 

 man in warm climates. 



