PRESERVATION  OF  LEECHES. 
223 
lure  of  white  and  red  lead,  the  part  of  it  which  is  exposed  to 
the  water  should  be  covered  with  shellac  dissolved  in  spirit,  and 
made  into  a  paste  with  chalk.  The  water  is  thus  prevented  from 
corning  into  contact  with  any  part  of  the  wood-work,  or  becoming 
contaminated  by  the  lead  cement.  The  cover  of  the  tank  should 
consist  of  a  wooden  frame  with  a  plate  of  glass  fixed  in  the  top, 
and  having  a  rim  with  a  piece  of  perforated  zinc  of  about  one 
inch  in  depth  let  in,  for  supplying  air.  This  cover  should  fit  on 
to  the  top  of  the  tank  like  a  shouldered  box,  so  as  to  prevent  the 
escape  of  the  leeches,  and  at  the  same  time  to  exclude  dust. 
Having  thus  prepared  the  tank,  some  coarse  river  sand,  well 
purified  by  washing  it  with  water,  is  to  be  mixed  with  small  peb- 
bles, and  this  mixture  is  to  be  introduced  so  as  to  cover  the  bot- 
tom of  the  tank  to  the  depth  of  about  an  inch.  A  water  plant 
is  now  to  be  selected.  I  have  used  the  Valisneria  spiralis,  but 
this,  not  being  a  native  of  this  country,  it  is  sometimes  difficult 
to  obtain,  in  which  case  some  other  water-weed  may  be  substitu- 
tuted  for  it,  such,  for  instance,  as  the  Anacharis,  which  chokes 
up  many  of  our  ponds  and  rivers.  The  roots  of  the  plant  are 
to  be  inserted  in  the  sand,  and  the  tank  is  then  to  be  filled  with 
clean  river  water.  The  next  requisite  is  the  water  snail,  two  or 
three  of  which  should  be  introduced  to  consume  the  decaying 
vegetable  matter  and  the  conferva  that  grows  on  the  sides  of  the 
vessel.  I  prefer  the  Plenorbis  corneus  for  this  purpose,  having 
observed  that  this  animal  devours  the  slimy  matter  given  off  by 
the  leeches,  and  does  not,  as  the  Limncea  stagnalis  does,  eat  the 
Valisneria  in  preference  to  substances  which  it  is  most  desirable 
to  get  rid  of. 
These  preparations  being  made,  before  introducing  the  leeches 
I  consider  it  better  to  let  a  week  or  two  elapse,  during  which 
time  the  plant  will  have  thrown  oiF  some  oxygen  with  which  the 
water  will  be  charged  ;  and  with  a  view  to  the  complete  fulfil- 
ment of  this  object,  it  will  be  found  advantageous  to  commence 
these  aquaria  in  the  spring  of  the  year  rather  than  the  autumn. 
The  best  situation  in  which  to  place  the  tank  is  in  a  room,  near  to  a 
window.  If  it  were  placed  in  too  strong  a  light,  such  as  it  would 
receive  in  the  open  air,  the  water  would  be  likely  to  become  tur- 
bid from  excessive  growth  of  conferva.  Even  under  the  condi- 
tions I  have  recommended,  the  glass  sides  of  the  tank  will  some- 
times become  more  or  less  opaque,  partly  from  the  growth  of 
