258 PLATHELMINTHES 



the worm thus produced escapes from the rest of the pilidium, which 

 quickly dies. 



Order I. Protonemertini. 



Nervous system outside the muscles; no stylets in the proboscis; mouth 

 behind brain. Carinella.* 



Order II. Mesonemertini. 



Nervous system in the muscles; mouth behind brain; no stylets. Cephalothrix.* 



Order III. Metanemertini. 



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

 rule with stylets. Geonemcrtes* and some species of Tetrastemn.a* terrestrial. 

 Am phi poms* (fig. 236, fresh water), Nectonemertes* Malacobdclla* leech- 

 like, with posterior sucker, parasitic in lamellibranchs. 



Order IV. Heteronemertini. 



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

 brain; proboscis unarmed. Linens* Micrura,* Cerebratulus* Zygeupolia* 



Summary of Important Facts. 



1. The PLATHELMINTHES are flattened bilateral animals with- 

 out coelom, whose nervous system consists of a supracesophageal ganglion 

 and lateral nerve trunks; the excretory system of branched protonephridia. 



2. The TURBELLARIA are the most primitive; the Trematoda and 

 Cestoda have descended from them. 



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

 no circulatory system. The digestive tract consists of ectodermal 

 pharynx and entodermal stomach, the latter many-branched in the 

 Polyclads, with three main branches in the Triclads, and rod-like in the 

 Rhabdocccles. 



4. Polyclads and Triclads are often united under the name Dendro- 

 coela. 



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 Distomitv there occur heterogony and alternation of hosts. 

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

 parthenogenetic eggs of which develop cercarias which become encysted 

 Distomiae in the second host, sexual Distomue in the third. 



7. Best known of the Distoma are D. hepaticurn and D. lanceolatum 

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

 man in warm climates. 



