THE XEMATOCYSTS OF EOLIDS 
135 
tocysts themselves. WTien touched and particularly when about 
to be cropped, the tentacles of a hydroid or of an actinian con- 
tract, and consequently fewer nematocysts are likeh^ to discharge 
than if the tentacle remained expanded. More to the point even 
than this is the fact that when tentacles are cut off quickly, no 
discharges, or only few occur. It follows, then, that eolids are 
not under a heavy fire to begin with. 
But such fire as they are under does not of necessit}' reach them, 
for, aside from the protection offered by the mucus which they 
secrete, the bodies of eolids are not the sort to be easily punctured 
by nettling threads. Certain experiments performed while study- 
ing the physiology of nematocysts ('09) show that a soft surface 
is apt to ward off the filament, and further that penetration is 
most easily effected when the thread is under its maximum speed 
at the beginning of the explosion. In order that a surface may 
be broken in upon, it must be very close to the mouth of the 
discharging nettle. Furthermore, Toppe ('09) has shown b}^ ex- 
tremely careful and interesting observations that the most com- 
mon type of hydroid nematocyst is especially adapted for punc- 
turing hard, chitinous surfaces and not soft indentible ones. 
That discharge of the nematocysts after ingestion does not 
take place, my own observations have shown to be untrue, never- 
theless such explosions are rare. In the paper on the physiology 
of nematocysts ('09) it was shown that the discharge of the fila- 
ment is really an explosion traceable to a rise in the internal 
pressure of the capsule. As this rise results normally from osmosis 
and as the fluids in the digestive tract of eolids are probably more 
concentrated than those inside the nematocysts, one can easily 
understand why explosions within the cerata should be rare 
enough to have hitherto escaped detection. 
Finall}^, it is not unlikely that immunity in the narrower sense 
renders ineffective such penetrations as may occur. The evidence 
of a" Reizgift "postulated to explain the irritations set up by nema- 
tocysts, does not appear to me as 3^et quite conclusive ('09). 
On the other hand, the evidence that there is present in nettles 
another poison, the hypnotoxin, responsible for the paralysis 
of punctured animals, seems well grounded. It is quite possible 
