ANIMALS AND PLANTS A SYMBIOTIC COMMUNITY. 255 
parties, precisely similar to that noticed in the case of lichens and in the others 
which have been described above. 
Several of the liverworts which live as epiphytes on the bark of trees exhibit 
on the under surface of their leaflets (which are inserted on the stem in two rows, 
and are pressed flat against the bark) little auricular structures, and in species of 
the genus Frullania, these take the form of definite hoods or pitchers. The rain 
that trickles down the trunks of the trees, washing the bark and wetting the liver- 
worts in its course, fills the hooded receptacles referred to with water, and is retained 
longer in these protected cavities than anywhere else, if a period of drought ensues 
and the liverwort becomes dry again. Now these cowls are the abode of tiny 
rotifers (Callidina symbiotica and C. Leitgebii), which live on the organic dust 
brought thither with the water. In return for the peaceful home thus afforded 
them in the hooded chambers of the leaves, the rotifers supply the liverworts in 
question with nitrogenous food. For as such must serve the matter excreted by the 
rotifers in the interior of the cowls. Without the intervention of the rotifers, the 
living organisms (Infusoria, Nostocine, and spores) contained in the water could 
not be converted into food by the liverworts, whereas the liquid manure arising 
from the Infusoria, Nostocinew, and spores, digested in the bodies of the rotifers, 
contains highly nitrogenous compounds, which are of great value to the liverworts 
in question, as indeed they are to all epiphytes living on the bark of trees. It 
stands to reason that the symbiotic liverworts and rotifers derive also a mutual 
advantage from the fact that the oxygen set free by the former comes into the 
possession of the rotifers and the carbonic acid emitted by the rotifers into that of 
the liverworts by the most direct method. 
Moreover, these cases of partnerships further remind us of other analogous rela- 
tions existing between plants and animals, which it is necessary to refer to now, 
although they cannot be treated in detail till later on. A great number of flowering- 
plants excrete honey into their flowers, and so attract flying insects to them, 
which supply themselves plentifully, and in their turn render to the plants they 
visit the service of transferring the pollen from flower to flower, thus making 
possible the development of fruits and fertile seeds. Certain small moths which 
- visit the flowers of Yucca bring the pollen to the stigmas, and force it into the 
stigmatic orifices in order that mature fruits and seeds may be produced from the 
rudimentary fruits, a result which is indeed a matter of vital importance to these 
moths. For the moths lay their eggs in the carpels of Yucca, and from the eggs 
larvee are developed which live exclusively on the seeds of this plant. If the 
Yucca were not fertilized, and did not develop any fruit, the larvee would die of 
hunger. A similar phenomenon occurs in many other cases of the kind, where 
both plant and animal reap some benefit. On the other hand, in the formation of 
galls, which are produced by animals laying their eggs in particular parts of plants, 
the advantage (with few exceptions) is all on the side of the animals, and these gall- 
structures might most justly be placed by the side of parasitic structures. 
It is obvious from all this that such of the mutual relations of plants and of 
