THE XEMATOCYSTS OF EOLIDS 
133 
also to understand why, as has been pointed out by Krembzow, 
the nucleus of the cnidophage takes no part whatever in the ''man- 
ufacture" of the nematocysts ; further the occurrence of individ- 
uals and of cerata devoid of nettles is readily explicable; and 
finally the identity of eolidian and ccelenterate nettles is no longer 
a case of convergence that exceeds the probabilities of homo- 
plastic evolution. However, some things remain to be explained. 
The first requisite for the development of these remarkable 
relations, is that eolids shall bt immune to the nettling organs of 
coelenterates. Since nudibranchs are animals of unusual delicacy 
and apparently without protection, the freedom with which they 
crawl over hydroids and actinians, and browse upon their heavily 
charged tentacles, is intelligible only on the assumption that the 
dangers in the midst of which the}' live do not appl}- to them. It 
is not without interest to inquire how this can be. 
The thought that most naturally comes to one is that in the 
course of time immunity has developed either by the elimination 
of individuals most prone to succumb to bombardment from nema- 
tocysts, or that in each generation the individuals, by being con- 
stantly under fire, gradually become indifferent to the punctures 
and stings, and finally fail entirely to react to stimuli to which the 
uninitiated respond by lively movements, and possibly by sensa- 
tions of pain. This reasoning, however, rests on the assumption 
not only that eolids are vulnerable, but that they are under fire, 
and both premises require qualification. 
That eolids are sensitive to the bombardment of nematocysts 
can be shown by transferring an animal which normally lives on 
Tubularia to the disc of a large Aiptasia. If the animal is not 
swallowed, it may fall off and reach the bottom of the dish in 
a perfectly rigid paralj^tic state. Occasionally spasmodic and 
angular movements may be observed, but recovery is rare as the 
animal usually dies despite many precautions. 
If the discrepancy in size be reversed, so that the eolid is con- 
siderably larger than the Aiptasia, the latter seems to affect to a 
marked degree only the highly sensitive foot-tentacles, and the 
eolid under certain circumstances may devour the actinian. The 
attack is made at the base, and as the anemone immediately re- 
