7O BOTANICAL GAZETTE [JULY 
like alga in another species of Convoluta.t? The color of the animal is due in part 
to orange-red glands in the superficial tissues, and in part to yellow-brown algal 
cells in the deeper tissues as well. The animals appear always to contain these 
yellow-brown cells, and in attempted cultures of the animals from the eggs, under 
conditions precluding infection by algae, the animals fail to develop. Further- 
more, when older animals, already containing numerous yellow-brown cells, are 
kept in the dark, the yellow-brown cells disappear, apparently being gradually 
reduced to indigestible granular remnants. Such animals, again lighted, fail to 
develop further unless reinfected; then, however, they grow rapidly as soon as 
the new algae have sufficiently multiplied. 
These facts indicate a dependence on the part of the animal upon its algal 
associates which may well be called parasitism, a degree of parasitism carried to 
the extreme of consuming the algae only when starvation impends, and under 
normal conditions falling far short of such destruction. In describing such an 
association as this, in which one animal becomes parasitic upon many much smaller 
algae inclosed within its own tissues, our usual vocabulary suffers strain, for 
words have to be used with altered meanings. Thus we should naturally speak 
but shall we speak of the infection of a parasite by its host? The relation of host 
and parasite in these tubellarians is like that of alga and fungus in lichens, the 
parasite incloses, lodges, and in time of hunger may ultimately devour its host. 
These convolutas secure their hosts by ingesting them with other food. The ani- 
mal feeds voraciously on a varied diet. Apparently in these animals, as in many 
others, digestion of great quantities of food is less complete than when only smaller 
quantities are taken at once. The brown algae, if ingested in small quantity, 
are all digested and destroyed, but if the quantity of food is large, some cells 
escape digestion, entering the tissue of the animal andin a way becoming a part of it. 
KEEBLE sees in the photosynthetic activity of these algal cells their value to 
the animals. The unicellular brown algae contain chromatophores in which 
12 See review in Bor. GAZETTE 46:68. 1908. 
