34 ROUSSELET : 



drop and mix, and then add again one drop more after every 

 five minutes for twenty minutes to half an hour, when the 

 polypides will appear fairly rigid and will no longer retract 

 when touched with a needle. Then add a pipette full of the 

 narcotic and wait another five minutes, when the animals will 

 probably be ready for killing and fixing. 



The killing is best done by adding a large pipette full of 

 formalin lo per cent, (commercial formalin lo parts, water 

 90 parts). 



After a quarter of an hour the extended and fixed Polyzoa 

 must be transferred into another bottle containing formalin 5 

 per cent., in which they are preserved permanently; fill up 

 the bottle to the top and in putting on the cork gently squeeze 

 out the superabundant fluid so as to close the bottle without 

 leaving an air bubble. 



Formalin 10 per cent, fixes and 5 per cent, preserves Poly- 

 zoa white and transparent ; it may bleach the tubes some- 

 what, but this is no great disadvantage. 



Very weak osmic acid may be used for fixing, but it stains 

 the animals more or less. 



The different species of Polj'zoa vary somewhat in their 

 behavior under the narcotic. Should a specimen not come out 

 after the first drop of cocaine, it will show that it has received 

 too much of it, and the colony must be removed to a bottle of 

 clean water and left there for some time to recover. 



I may mention that it is of no use applying a dose of the 

 narcotic and allowing the Polyzoa to die in it ; the polypides 

 will simply curl up and become macerated and show no struc- 

 ture when fixed. 



When collecting Polyzoa, the statoblasts, which are very 

 characteristic of the various fresh water species, should always 

 be searched for and preserved at the same time. 



Poh^zoa are found attached to the stems and leaves of 

 aquatic plants in lakes, canals, water reservoirs, backwaters 

 of rivers and slowlv movine streams ; also on submerged 



