CORRIGENDA 
Page 212, second paragraph, last line, after “equally bad” insert: 
“and snails are common during the rainy season ,” 
Page 215, Legends to Figs. 1—4, the magnifications should be altered to read: 
Fig. 1, x 115; Fig. 2, x230; Fig. 3, x230; Fig. 4, x485. 
