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 offan 
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 
