A striped polyclad flatvvorm, Prostheceraeus vittatus, 

 about H/k inches long, glides over weeds or under 

 stones on marine shores. The head, at top, bears sen- 

 sory projections. (Enghind. D. P. Wilson) 



often 1 or 2 inches long. Many have a pair of sensory 

 tentacles on the head, and two or more clusters of 

 minute eyes. Numerous eyes may also be scattered 

 over the front end or all or part of the body margin. 

 Warm-water polyclads, especially those of coral 

 reefs, may be richly colored ( Plate 35 ) . Others are 

 striped in strongly contrasting colors. Even many of 

 the white, gray, or brown ones of temperate waters 

 delight the eye with their translucency, their ele- 

 gantly ruffled edges, and their graceful undulations 

 when they take off on a brief swim through the water. 

 Pelagic species, which drift or swim, are usually 

 transparent or translucent and are found down to 

 three thousand feet, as well as at the surface. Some 



live in the open sea only by clinging to floating sar- 

 gassum seaweed. None of the polyclads is a parasite, 

 though a number are supposedly harmless commen- 

 sals, like Hoptopkma ifiqitiliiw, which lives in the 

 mantle chamber of the big marine snail Busycon, on 

 the American Atlantic coast. The oyster leech, Sty- 

 locluis fmntalis. does serious damage to oyster beds 

 in Florida, and it has close relatives that prey on 

 oysters of both American coasts. Some members of its 

 family are the largest American polyclads, broad 

 worms over 2 inches long. 



The Flukes 



(Class Trematoda) 



The flukes take their Anglo-Saxon common name 

 from their flat shape, and the technical name of the 

 group comes from the Greek word trema, "a hole," 

 which refers to the cavity of the suckers by which 

 these small, often leaflike, exclusively parasitic 

 worms attach to their human or other vertebrate 

 hosts. Some have sharp hooks to supplement their 

 suckers or other adhesive organs. The adults have 

 lost the ciliated epidermis, the outer layer of cells 

 that covers their turbellarian relatives. In its place is 

 a thin cuticle secreted by underlying cells. Flukes, 

 also called trematodes, are structurally simple in most 

 respects, but the reproductive system, which occu- 

 pies most of the animal's interior, is something else 

 again. In complexity it equals any to be found in the 

 higher animals, and its efficiency makes these lowly 

 parasites truly formidable contenders for man's blood 

 and other living tissues. 



THE MONOGENETIC FLUKES 



The monogenetic flukes are so labeled because 

 they have a simple life history, with only one host. 

 Of about seven hundred species described up to now, 

 most are external parasites that live on the gills, or 

 sometimes on the skin, of both fresh-water and ma- 

 rine fishes, feeding on the external covering layer or 

 on oozing blood. In nature they seldom do much 

 harm. But when man steps into the picture, provid- 

 ing special fish hatcheries where young fishes are 

 raised in dense concentrations, these external flukes 

 can cause serious losses of fish. 



Hanging onto the outside of a fast-moving fish is 

 not accomplished without a really tenacious hold, 

 and Gyrodactylus fastens onto the gills of its hosts 

 with a well-developed adhesive disk at the rear end. 

 The center of the disk has two or four large hooks, 

 and its periphery is bordered with small hooks. As 

 they loop about on the surface of the host, external 

 flukes inevitably stray occasionally into the cavities 

 which connect with the exterior: the mouth, the nasal 

 cavities, and the urinary bladder. In the course of 

 much time, some of the monogenetic flukes have be- 



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