142 LIFE : OUTLINES OF GENERAL BIOLOGY 



ciliated, multi-nucleate saprophytic Opalina of the frog's rectum, 

 and (perhaps) also the many predatory lorms, like the Trypano- 

 somes of sleeping sickness. 



There are no parasitic sponges, though many are epiphytic or 

 epizoic. Among Coclentera there are but a few instances; thus a 

 polyp [Polypodium hydrifortfie) occurs on the ova of the sterlet 

 {Acipenser ruthenus). More than one medusoid {Cunina parasitica, 

 etc.) are found in close nutritive dependence on other members of 

 their group. The e.xtremely simple and thus perplexing Mesozoa, 

 Dicyema, and its allies, are all parasitic; some of them arc 

 possibly extremely degenerate worm-tyix?s. 



Fig. 37. 



Bilharzia or Schistosotmtm hctmatobium. After Looss. The male, about three- 

 fifths of an inch long, carries the longer female (F) in a groove (GC); 

 S, the suctorial mouth; I*S. the ix)stcrior adhesive sucker; R, part of 

 the reprotluctive system of the female. 



In the phylum or series of flat-worms (Platyhelminthes), a few 

 members of the class Turbellaria are parasitic, one in marine Molluscs, 

 another in or on Holothurians, The members of the allied class of 

 Trcmatodes are all either ectoparasites or endoparasites. The rare 

 Temnocephalids adhere to freshwater crayfishes, crabs, turtles, etc.. 

 but do not feed on their hosts. This Platyhclminth series has its 

 bathos in the Cestodes, which are all cndoparasitic, both in their 

 encysted (bladderworm) and adult (ta|>cworm) stages. All the 

 Nemerteans are free-living, unless two or three found on crustaceans 

 can be called parasites. The aberrant and leech-like Malacobdella 



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