122 NATURALISTS' ASSISTANT. 



filled. In Vertebrates the arterial system may be injected 

 from the aorta. In Crustacea a small hole should be bored 

 through the carapax immediately over the heart and into this 

 the nozzle of the syringe should be inserted, taking care that 

 the tip enters, but does not pass through the heart. Injecting 

 mollusks is rather more difficult, the injection should be 

 forced into the heart, or into the vessel at the base of the 

 gills ; while starfishes are most easily injected by cutting off an 

 arm and inserting the nozzle into the tube found on the lower 

 surface. Insects and worms are only injected with very 

 delicate manipulation. The method just described injects 

 only the arterial system. To fill the veins is not so easy. It 

 is best accomplished by forcing the injection into some distal 

 portion and allowing it to find its way back to the heart. 

 Thus the web of a frog's foot, the claw of a lobster and the 

 foot of a mollusk are the best places for those forms. Great 

 care must be exercised in performing an injection that the 

 pressure employed is not sufficient to rupture the vessels. 

 The more recently life is extinct the stronger the vessels are. 

 Besides injecting colored material to aid in the demonstra- 

 tion of the circulating system, various preservations are some- 

 times injected into the arteries, alimentary canal and body 

 cavity to aid in keeping the specimens. Herr Wickerschei- 

 mer recommends "his fluid for this purpose. 



SECTION CUTTING. 



It is necessary in making microscopic examinations of 

 structures and tissues to have them thin enough to be trans- 

 parent, or very translucent, otherwise they cannot be well 



