594 
If,  however,  living  larvae  are  placed  in  the  water  there  is  a  verv 
rapid  rise  in  the  number  of  bacteria  per  c.c.,  which  is  enormously 
increased  if  a  larva  happens  to  die.  In  other  words,  larv^ae  add  some¬ 
thing  to  the  water,  probably  mucus,  which  acts  as  food  material,  and 
which  therefore  increases  the  rate  of  development  of  the  bacteria, 
and  a  dead  larva  in  decomposing  still  further  increases  the  bacterial 
proportion. 
In  the  case  (Series  IV)  where  typhoid  bacilli  were  added  to  the 
water,  the  presence  of  the  larvae  did  not  appear  to  have  the  least 
effect  in  reducing  their  numbers  ;  on  the  contrary,  the  total  number  of 
all  bacteria  went  up.  It  is  known  that  typhoid  bacilli  do  not  tend  to 
multiply  in  clean  drinking  water,  but,  on  the  other  hand,  the  presence 
of  albuminous  material  such  as  would  be  derived  from  the  bodies  of 
larvae  might  tend  to  favour  their  multiplication. 
The  experiments,  therefore,  show  that  the  presence  of  larvae  in 
drinking  water  adds  very  considerably  to  the  number  of  bacteria 
present.  In  the  tropics,  the  number  of  larvae  to  be  found  in  drinking- 
water  receptacles  is  often  very  great,  and  amongst  the  larvae,  there 
will  usually  be  found  some  which  have  perishctl  ;  therefore,  we  may 
reasonably  conclude  that  the  bacterial  content  of  such  water  is  very 
high  indeed.  If  larvae  lead  to  the  increase  of  saprophytic  bacteria, 
it  is  reasonable  to  suppose  that  they  will  not  diminish  pathogenic 
forms.  The  evidence,  therefore,  strongly  points  to  the  fact  that 
larvae  in  water  will  still  further  pollute  it.  The  observations  upon 
Cyclops,  as  far  as  they  go  also  point  in  the  same  direction. 
