702 



ECOLOGY AND EVOLUTION 



aflFords a good illustrative example. Bird 

 malaria is adjusted to birds and cuUcine 

 mosquitoes in a similar type of life cycle. 

 Many instances of cyclomorphic popu- 

 lations adjusted to a number of aspects of 

 the community are found among the 

 parasitic worms— flukes (Fig. 249), tape- 

 worms (Fig. 250), nematodes (Fig. 251), 

 and acanthocephalans (Thomas, 1944). In 

 many cases two hosts are involved, and in 



host and enter either another snail of the 

 same or a diflFerent species, or some other 

 invertebrate such as an insect larva, and 

 encyst. In some cases, the cercarias encyst 

 within the redia or sporocyst without leav- 

 ing the first host. Some species are trans- 

 mitted passively to a second or third inter- 

 mediate host. The cercarias of some i ami- 

 lies (Opisthorchidae, Heterophyidae, Stri- 

 geidae) invade the skin of fishes or amphib- 



Fig. 249. Life cycle of a fluke (Halipegus eccentricus) . Eggs passed in the feces of the frog 

 ( 5 ) are eaten by a snail, Physa or Helisoma ( 1 ) . The eggs hatch ( a ) and become sporocysts 

 (b), each of which develops three or more rediae (c). Within a month each of the rediae has 

 50 or more cercaria (d) that may be eaten by Cyclops (2), in which they develop into meso- 

 cercaria in the body cavity. The tadpole (3) sucks up the infected Cyclops. The young flukes 

 ( / ) migrate from the stomach to the mouth ( g ) , and the adult fluke ( h ) finally migrates to the 

 auditory tube of the adult frog (5). (From Thomas.) 



some instances three or even four. The 

 first intermediate host of all digenetic flukes 

 is a mollusk, except in Cercaria loosii, 

 which infects the marine anneUd, Hyd- 

 roides hexagonus, of the Atlantic coast of 

 the United States (Martin, 1944). 



Digenetic trematodes show a variety of 

 types of life cycles. The cercarias of some 

 (Fasciolidae, NotocotyUdae, Paramphisto- 

 inidae), on leaving the first host, encyst in 

 the open and survive only if ingested by 

 a suitable final host. Others (Echinosto- 

 matidae, Lepodermatidae ) leave the snail 



ians and encyst in the host tissues, there 

 to be later ingested by a final host. The 

 cercarias of the blood flukes ( Scliistosomati- 

 dae) leave the molluscan host and pene- 

 trate the skin of a fish, bird, or mammal 

 directly, and migrate to the host circulatory 

 system (Bartsch, 1946). 



Such types of Ufe cycles may have 

 evolved one from the other. A possible 

 free-living adult resembling a cercaria may 

 have been characteristic of the ancestral 

 types (Bayhs, 1938). Baer (1933) ob- 

 served that the number of genera of trema- 



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