This paper introduces a kind of vacuumizing system for X-ray machine head.The system function,structural composition and operation principle are expounded.It has such advantages as small empty volume,high vacuumizing speed,simple and convenient structure,etc.It shows high military,social and economic benefit because of its good maneuverability in timely and efficient maintenance on the spot.According to the feedback from maintenance department of field and local medical equipment,it is stable and reliable in working,easy to be operated and worthy of being popularized.

Objective To understand the successive dynamic change of population structure of Oncomelania hupensis during a one-year period,so as to provide the evidence for snail control. Methods A river beach and a ditch infested with O. hupensis snails were selected and longitudinally investigated in the midmonth during one year. The snail survey indices included the sur-vival status,gender,number of whorls,length and width of shell,and gonad development status(measured by the color depth of gonad and the length ratio of gonad to liver),and the monthly snail eggs in the soil were collected and counted simultaneous-ly. In addition,the temperature and humidity of the soil and the daily data of air temperature and precipitation were measured or collected during the study period(every month). Results Both survival rate of snails and live snail density at the two environ-ments were positively correlated with the temperatures of air and soil. With a slight bimodal distribution ,the snail survival rate peaked from May to June,and in September. The living snail densities got the highest level in July and September in the river beach,and from April to May in the ditch. The regression equations of snail length(L)and width(W)were Lbeach = 2.355 +1.678W(F=2989.43,P<0.01)and Lditch=0.478+2.091W(F=2.989.43,P<0.01),respectively. The snails were the ones with 4.07-11.81 mm in the length(8.98 ± 0.92)mm in the river beach,and the snails were the ones with 3.63-9.92 mm in the length(7.03 ± 0.82)mm in the ditch. The main snails were the ones with five to eight whorls of shell in the river beach and four to seven whorls of shell in the ditch. The proportions of snails with less than or equal to five whorls(in the river beach)and four (in the ditch)were the highest in May and September,about 20％. The ratios of male and female snails were 1.66 in the river beach and 1.22 in the ditch,respectively. The gonad development status of male and female snails was basically synchronous and had a bimodal abundance period-from April to May and September to October. The numbers of snail eggs in the soil among months were significantly different,reaching the highest in June in the river beach(100.8/0.1 m2),and May in the ditch(82.5/0.1 m2). Conclusion The principal periods of breeding and alternation of generations of snails are April-May and September-October every year,which should also be the optimal time for mollusciciding in schistosomiasis susceptible zones.