of Edinburgh, Session 1881-82. 
387 
Thus, then, for a vegetable cell no more ideal existence can be 
imagined than that within the body of an animal cell of sufficient active 
vitality to manure it with abundance of carbonic anhydride and nitro- 
genous waste, yet of sufficient transparency to allow. the free entrance 
of the necessary light. And conversely, for an animal cell there can 
be no more ideal existence than to contain a sufficient number of 
vegetable cells, constantly removing its waste products, supplying 
it with oxygen and starch, and being digestible after death. For 
our present knowledge* of the power of intracellular digestion 
possessed by the endoderm cells of the lower invertebrates removes 
all difficulties both as to the mode of entrance of the algse and as to 
its fate when dead. In short, we have here the economic inter- 
relations of the animal and the vegetable world reduced to the 
simplest and closest conceivable form. 
It must be by this time sufficiently obvious that this remarkable 
association of plant and animal is by no means to be termed a case 
of parasitism. If so, the animals so infested would be weakened, 
whereas their exceptional success in the struggle for existence is 
evident. Antmea cereus , which contains most algae, probably far 
outnumbers all the other species of sea-anemones put together, and 
the Kadiolarians which contain yellow cells, are far more abundant 
than those which are destitute of them. So, too, the young gono- 
phores of Yelella, which bud off from the parent colony, and start in 
life with a provision of Philozoon (analogous to, yet far better than, 
a yolk-sac), survive a fortnight or more in a small bottle, — far 
longer, so far as my observations go, than any other small pelagic 
animals. So, too, a Ehizostome medusae like Cassiopeia , which is 
well provided with Pliilozoon , lives for w T eeks in an aquarium, while 
Pelagia , which has no algae, dies in a day or two. Anthea, too, can 
be exposed to light all day in stagnant water without apparent 
inconvenience, but dies if left in it over night. Such instances, 
which might no doubt easily be multiplied, show how beneficial the 
association is to the animals concerned. 
The nearest analogue to this remarkable partnership is found in 
the vegetable kingdom, where, as the researches of Schwendener, 
Bornet, Stahl, and others have shown, we have certain algse and 
* Krukenberg, Z. Kritik dcs Schriften ueb. sog. intracell. Verdauung b. Cod , 
Vergl. Physiol. Studien, ii., 1 Abth., 1882, p. 139. 
VOL. XI. 3 D 
