330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



migrate for any appreciable distance, for their food supply from 

 fallen and broken coconuts is always plentiful. The rat acts as a 

 blind reservoir host, catching the disease in the creeks, but apparently 

 not passing schistosome eggs in their feces. A glance at a smear prep- 

 aration of the liver under the microscope will show whether the rat is 

 infected or not. In temperate regions of China and Japan the rat 

 method is usually unsuccessful for locating endemic areas, for the 

 animals prefer to remain near houses and barns and are not as likely 

 to become infected. 



Control measures against the snail were undertaken jointly by the 

 Army and Navy. As a preliminary to chemical tests, the life history 

 of the snail had to be known, for eliminating all visible snails might 

 prove useless if, unknown to the workers, a habit of migration or 

 secluded aestivation existed, or if the eggs of the snail could resist 

 poisons. Two types of experiments were carried out to determine if 

 the snail had a habit of migrating. The first was simply by setting 

 a marker in a creek and liberating 500 snails whose shells had been 

 painted bright yellow. Two weeks later 90 percent of the marked 

 snails were recovered within 25 feet of the marker. Other observa- 

 tions involved the method of parasitological examination. At the edge 

 of a marsh on the south side of the town of Palo a drainage ditch 

 empties its schisto-polluted filth into the area of an extensive colony 

 of Oncomelania snails. At the mouth of the ditch, where schistosome 

 eggs were hatching into miracidia by the thousands, it was found upon 

 microscopic examination that the snails were heavily infected with 

 cercariae. Snail samples taken progressively farther from the ditch, 

 and hence in zones of less miracidial exposure, showed lower and lower 

 percentages of infection. Since the lapsed time from miracidial pene- 

 tration to cercarial development is 8 to 9 weeks, it was safe to assume 

 that the snails do not voluntarily migrate more than a few feet for 

 at least that period of time. 



The search for the egg of the Oncomelania mollusk lasted for 3 

 months. The size and nature of the egg were unknown to the Navy 

 workers, and females kept in captivity could not be induced to lay. 

 MoUuek eggs of a dozen species found in the creeks were carefully 

 raised to an advanced stage of development so that identification 

 could be made, but in every case they were found to belong to other 

 species. The thrill of finally finding Oncomelania snail eggs was one 

 which only comes to a naturalist engrossed in such strange searches. 

 The egg was not only of extremely small size, no larger than the head 

 of a pin, but was always carefully camouflaged by the feces of the 

 female. The excrement of this snail is made up almost entirely of 

 fine bits of undigested grit and sand. As the egg leaves the oviduct 

 and is stuck to the surface of a moist piece of wood or coconut shell, 

 the female places a number of her sandy fecal pellets on the egg and 



