388 Proceedings of the Royal Society 
fungi associating themselves into the remarkable colonies known as 
lichens, so that we may not unfairly call our agricultural Radio- 
larians and Coelenterates Animal Lichens, an alliance as to the 
possibility of which Semper * has already speculated. And if there 
he any parasitism in the matter, it is by no means of the alga upon the 
animal, hut of the animal, like the fungus, upon the alga, and this 
utilisation of plants by animals contrasts curiously with the inverse 
case of the utilisation of animals by vegetables shown us by carni- 
vorous plants. Yet the association of Anthea , &c., with Philozoon , 
exhibits a far more complex balance and interaction than either 
lichen or insectivorous plant, and stands unique in physiology as the 
highest development of the reciprocity between the animal and the 
vegetable kingdom. 
Thus, then, the list of supposed chlorophyll-containing animals 
with which we started breaks up into three categories : — first, those 
which do not contain chlorophyll at all, but green pigments of 
unknown function ( Bonellia , Idotea , &c.) ; secondly, those vege- 
tating by their own intrinsic chlorophyll (Hydra, Spongilla , 
Convoluta ) : thirdly, those vegetating by proxy, if one may so speak, 
rearing copious crops of algse in their own tissues, and profiting in 
every way by their vital activities. 
It might be objected that the chlorophyll of Convoluta, &c., is 
also derived from some alga more highly modified than our Philo- 
zoon by sojourn within the tissues of the animal. Only an embryo- 
logical investigation would of course furnish absolute proof of the 
animal nature of chlorophyll ; but pending this, I was enabled, 
through the kindness of M. de Lacaze-Duthiers, while returning 
home through Paris in November, to re-examine specimens of 
this Planarian. Its green cells, however, bear not the slightest 
resemblance to Philozoon, or any other alga known to me ; are not 
irregularly scattered, but form an almost continuous definite layer 
in every individual; are not of definite shape, nor bounded by any 
cellulose coat, but are irregularly ellipsoidal, semi-fluid, naked ; are 
rarely, if ever, to be seen multiplying by transverse division, and 
do not survive the death of the animal for any length of time. It 
is also evident, on the most superficial examination, that the chloro- 
* Animal Life (Internat. Sci. Series), Lond., 1881, p. 74. 
