droids may be killed in this way fully expanded. When the 

 second method is followed anaesthesia may be induced in 

 various ways. Earthworms soon become unconscious and 

 perfectly relaxed in dilute (30-50'/' ) alcohol, and may be 

 killed in strong alcohol. The most generally useful anaes- 

 thetic is the vapor of chloroform. Specimens should never 

 come in contact with the liquid. Marine animals are very 

 easily and successfully killed by immersion in fresh water 

 and vice versa. 



Fixing. As soon as an animal dies decomposition pro- 

 cesses begin to alter the structure of the tissues and in order 

 to prevent these changes the natural structure must be 

 "fixed." Nearly all sudden killing fluids fix tissues at the 

 same time. After the use of anaesthetics, however, tissues 

 should be fixed as soon as possible by immersion in the pres- 

 erving medium. Successful fixation depends on bringing all 

 the tissues quickly in contact with the fixing agent. To facili- 

 tate this an incision may be made to admit the fluid into large 

 specimens. Fixation is synonymous with hardening and dur- 

 ing the process specimens must be held in the position in 

 which they are wanted permanently. 



Preserving. In biological practice two preserving media 

 are in general use. Tissues may be kept indefinitely in 70% 

 alcohol or, if they are thoroughly fixed, even in the vapor of al- 

 cohol. Fresh tissues, however, contain much water and should 

 be immersed in several times their bulk of full strength alcohol, 

 which must be renewed after a few days if the specimens 

 are large. Formalin is sold in 40/' solution, 5-7V' of which 

 in water is sufficient to preserve animals as large as a cat. 

 For smaller forms 1-4% formalin may be used. Formalin 

 is much more economical than alcohol, does not extract so 

 much water from the specimens, and does not evaporate so 

 quickly and should therefore be used for preservation of 

 all large animals. For smaller forms alcohol is probably 

 simplest to use. 



Summary. Small animals should be killed, fixed, and 



16 



