General Ecology 21 



THE PLANORBIDAE AS HOSTS FOR PARASITIC WORMS 



The family Planorbidae furnishes some of the best examjiles of the role of 

 snails in the life history of certain trematode worms known as flukes. A 

 group of worms known as blood flukes {Schistosoma) cause serious dis- 

 turbances in man. Three species are known from different parts of the 

 world. These parasites require a mollusk as the host of the parthenogenetic 

 generations of their life cycle. In this life cycle the Schistosoma in the 

 human host lays eggs which are voided in the faeces or the urine. Eventu- 

 ally the larvae in the eggs become clothed with a ciliated epithelium, and, 

 on coming into water, the egg shell bursts and the miracidium escapes, 

 whereupon it begins to swim about rapidly (Faust, 1932. p. 350). 



These miracidia are adapted to specific groups of snails. When in their 

 swimming they reach the proximity of certain snails they show a specific 

 response to that species of mollusk, probably due to a mucous secretion of 

 the animal. In most cases they readily attack, and, in the course of half 

 an hour or an hour, penetrate the soft tissues of the snail. In certain cases 

 of other trematodes iClonorchis and Metagonimiis) , however, it is neces- 

 sary for the embryonated eggs to be swallowed by the snail, whereupon 

 they hatch in the lumen of the oesophagus, bore their way through the wall, 

 ancl gradually migrate through lymph channels into the lymph spaces 

 bathing the hepatic glands. In the meantime, they have metamorphosed 

 into the simple sacculate sporocysts. These first generation sporocysts give 

 rise to a parthogenetic progeny (either second generation sporocysts or 

 rediae) . From this second generation there develops (also parthogeneti- 

 cally), a progeny of larvae known as cercariae. Thus, from the penetration 

 of a single miracidium there may be developed as many as ten thousand 

 or more cercariae. Then later, on reaching maturity, they erupt from the 

 tissues of the snail, and, either by direct or indirect routes, produce 

 infection in human or reservoir hosts (from Faust, 1932, pp. 350-351). 



Although the three human species of blood flukes (Schistosoma) are 

 closely related, they utilize entirely different species of mollusks for the 

 l)arthenogenetic stages. Thus, Schistosoma haematobium, with a distribu- 

 tion in northern Africa, the Congo Basin, the Sudan, ^Mesopotamia, 

 Palestine, Sicily, Corsica, and possibly southern France, parasitizes species 

 of the genera Bulinus and Physopsis. Schistosoma japonicum, with a distri- 

 bution in China, Japan, Formosa, and the Philippine Islands, uses mem- 

 bers of the genus Oncomelania. 



The third species. Schistosoma mansoni, occurring in the same localities 

 in Africa as S. haematobium, is also found in northern South America and 

 many of the West Indian islands whither it was brought from Africa by 

 slaves during the days of the slave traffic. This species originally para- 

 sitized small planorbids belonging to the genus now known as Afropla- 

 norbis. In the West Indies a genus of large snails known as Australorbis is 

 parasitized by the Schistosoma mansoni, which has become a serious pest 

 in these regions. A genus of smaller snails, Tropicorbis, related to the 

 African group Afroplanorbis, is also used as intermediate host. The small 

 snails of the genus Drepanotrema have also been found with cercariae. 

 Attempts to use the genus Helisoma as a host for Schistosoma mansoni 



