204 SECTIONAL ADDRESSES. 
latter case the symbionts were found capable of surviving their host, 
of multiplying, and of assuming a flagellate stage. 
Zoochlorella occur mainly in fresh-water animals, zooxanthella 
mainly in marine animals, the symbionts, measuring 3-10 microns in 
size, being found in many Protozoa, Sponges, Cceleriterates, Cteno- 
phores, Turbellaria, Rotifers, Bryozoa, Annelids and Molluscs. 
Physiological relations between Animals and Symbiotic Alg@.—In 
1879, Geddes showed that green animals give off oxygen, Convoluta 
roscoffensis (Turbellaria), when well illuminated, liberating gas con- 
taining 45-55 per cent. of oxygen. Engelmann (1881), by means of 
his bacteria-method, showed that Hydra viridis (Ccelenterata) and 
Paramecium bursaria (Protozoa) give off oxygen when exposed to light. 
Geddes (1882), working with a series of marine animals, found Velella 
gave off 21-24 per cent. of oxygen, and an Actinia (Anthoa cereus) gave 
off 32-38 per cent. of oxygen. Whereas animals harbouring green 
alge as symbionts always liberated oxygen, the colourless varieties of 
these animals never did so. Geddes regarded the association of animal 
and alga as being mutually helpful, the oxygen supplied by the alga 
to the animal and the carbon dioxide and nitrogen supplied by the 
animal to the alga being useful to the partners. He speaks of ‘animal 
lichens ’ and ‘ Agricultural Radiolarians and Celenterates.’ He found, 
moreover, that animals harbouring symbionts are much more resistant 
than those without symbionts: Meduse (Velella) survived 14 days in 
small beakers with symbionts, only 1-2 days without them. Proto- 
zoologists have, moreover, found that Protists harbouring symbionts 
are easier to rear in vessels than are those without symbionts. Brandt 
(1883) believes that the symbionts and host aid each other in nutrition. 
Green Spongilla (fresh-water sponges) and Hydra viridis may live a 
long time in filtered water. He found that when starved green Actinia 
were (a) placed in the dark, they expelled their alge and died rapidly, 
being probably poisoned by the dead algee, but that when they were (b) 
placed in diffuse light they lived on. Actinia deprived of symbionts 
may become habituated in culture to live without them. Opinions 
(vide Buchner, 1921) are in conflict as to the exact relationship 
between the partners; in some cases (Peneroplis and Trichospherium) 
the symbionts never appear to be injured, in Ameba viridis, &e., a 
limited number of symbionts are digested at all times, whereas in some 
Radiolarians, &c., digestion only takes place at certain stages of their 
development. Nutritive substances pass from the algze into the host’s 
cells; thus starch granules, found alongside the alge, may be taken up 
by the animal cells. 
Using modern methods of gas analysis, Trendelenburg (1909) con- 
cludes that green Actinias (Anemonia sulcata) live in true symbiosis 
with alge, the algee supply oxygen to the animal by day and at night 
utilise the surplus oxygen evolved, whilst carbon dioxide is furnished 
to the alga partly by the animal and partly by the water in which they 
are bathed. Riitter (1911) studied the nitrogen metabolism and con- 
cludes (a) that the Actinia yields to the alge nitrogen in the form of 
ammonia for protein synthesis, and in darkness it adds carbon contain- 
ing substances (nitrogen-free), whilst (b) the alge yield to the Actinia 
km 
