PARKER: THE REACTIONS OF METRIDIUM. 111 
the soluble constituents of the food. That the latter is the true ex- 
planation is shown by the fact that, when filtered meat juice is gently 
discharged over the tentacles of Metridium, they respond in precisely 
the same way as though they were in contact with solid food, a reaction 
which does not follow a similar discharge of sea-water. Moreover, iso- 
lated tentacles can be thrown into active contractions if, while gliding 
through the water, a drop of meat juice be discharged over them. 
These facts confirm Nagel’s (’92, p. 335) conclusion that the soluble 
constituents of the food stimulate the tentacles of actinians. So far as 
these relations are concerned, Nagel distinguishes three classes of sub- 
stances: first, materials, like sea-water, to which the tentacles are in- 
different; secondly, substances, like meat, to which the tentacles attach 
themselves ; and, thirdly, reagents, like quinine, from which the tentacles 
retract as from a harmful substance. So far as Metridium is concerned, 
I have been able to distinguish only two of these classes, namely the 
first and second. To the second class belong meats and meat juices, and 
probably all the forms of food taken by the animal; the application of 
this material to the tentacles produces a temporary reversal of them, 
so that these organs point toward the mouth instead of away from it. 
To the first class belong such substances as powdered carmine in sea- 
water. Toward such materials the tentacles seem to be indifferent ; 
thus, if carmine be placed upon quiescent tentacles, it will be discharged 
over the edges of the oral disk without disturbance. If, during such an 
operation, a piece of meat be put on the tentacles, the latter will respond 
as usual, and more or less carmine together with the piece of meat will 
be carried into the mouth. Ifa piece of meat be placed on the tenta- 
cles and well started toward the mouth, a quantity of carmine will not 
reverse the action, for the carmine with the meat will be carried into 
the mouth. Thus the tentacles are indifferent to the carmine and re- 
spond to the meat only. 
To the same class as carmine belong India ink, sand, small pieces of 
rubber, pellets of filter paper softened in salt water, etc. Nagel (92, p. 
335) states that the tentacles of Adamsia respond to sugar in the same 
way as to weak meat juice. In Metridium I have never succeeded in 
getting any response from the tentacles to either sugar dissolved in sea- 
water or sugar crystals; nor have I ever noticed any retraction of the 
tentacles when bathed with a strong solution of quinine or with a weak 
solution of picric acid, or when brought in contact with pellets of paper 
containing quinine. To all of these substances the tentacles were as 
indifferent as to carmine in water. 
