82 PLATYHELMINTHES 



General Considerations 



Distribution. — Free living flatworms occur from the deep sea to 

 the surface of fresh water lakes, while parasitic forms infest animals 

 of all the higher Phyla. 



The Turbellaria are usually free-living, consuming insect larvae 

 and aquatic worms and eking out their diet by means of diatoms and 

 algae. Some forms like the flatworm, Bdelloura, which lives in the 

 gill books of the king crab, are commensals, while still others are 

 found parasitic in the intestines of Echinoderms and worms. 



The Trematoda are all parasitic, some of them attaching to the 

 gills of fishes by hooks and suckers as ectoparasites. Others are 

 true internal parasites, found in the pericardial cavity of the mussels, 

 the urinary bladder of Amphibia and the alimentary canals, liver 

 and lungs of vertebrates. Many Trematodes find molluscs neces- 

 sary as their secondary hosts, the commonest instance being that 

 of the liver-fluke and the snail. 



The Cestoda are internal parasites usually found in the ali- 

 mentary canal and requiring another vertebrate or invertebrate as 

 a secondary host. Passage from one host to another is not an active 

 migration as in the Trematodes. On account of their extreme 

 parasitism we find that the Cestodes are the most degenerate of the 

 flat worms. 



Physiology. — The Turbellaria are covered with fine vibratile 

 cilia which aid in respiration as well as locomotion. They have a 

 well-developed, branched digestive tract, and a complicated excre- 

 tory system consisting of water vessels which give off fine capillaries, 

 which terminate in flame cells. It is assumed that the excretory 

 system may also function in respiration. The reproductive system 

 is " monoecious," or hermaphroditic? The nervous system is 

 highly developed, consisting of central cerebral ganglia or brain, 

 from which proceed posteriorly two longitudinal ventral nerve 

 cords, with connecting nerve strands or commissures. The Trema- 

 toda lack cilia but have minute cuticular papillae. They have an 

 anterior mouth surrounded by a muscular oral sucker, while poste- 

 riorly is a larger ventral sucker. Other openings are the median 

 genital openings and the posteriorly situated excretory pore. The 

 mouth leads into a muscular pharynx, a short esophagus and a rather 



"^ Consult Curtis, W. C. 1902. Life history, normal fission, and reproductive 

 organs of Planaria maculata. Bost. Soc. Nat. Hist., vol. 30. 



