YELLOW-BROWN GELLS OF CONVOLUTA PARADOXA. 467 
reflected, black-brown by transmitted light, which occur in the 
body of C. paradoxa consist probably of urates (PI. 26, figs. 
4and 5). The granular substance of which these bands are 
composed is always present, although the amount varies very 
considerably. Even in the just-hatched larva the bands are 
indicated’as grey-black patches formed by groups of sparse 
granules. As the animals mature the amount of granular 
substance increases, though at the time of egg-laying it may 
disappear almost entirely. Now the hypothesis that the 
yellow-brown cells may utilise uric acid for their proteid- 
synthesis presents no difficulty. For instance, it has been 
established that holoplytic plants may utilise for this purpose 
organic nitrogen in many different forms, e.g. urea, uric acid, 
asparagin, leucin, tyrosin, guanin, kreatin, hippuric acid, etc., 
and, moreover, it has been shown that uric acid may replace 
nitrates in water-cultures of the higher green plants (Pfeffer, 
1900). 
The problem of the luxuriant development of the yellow- 
brown cells in the body of C. paradoxa may be thus stated 
in terms of the nitrogen-hypothesis :—C. paradoxa contains 
stores of waste organic nitrogen, presumably in the form of 
urates. Such substances are known to serve as sources of 
nitrogen to various plants. Is the yellow-brown cell able to 
utilise such substances ? 
In order to answer this question two modes of experimen- 
tations were adopted. The first method was as follows :— 
Batches of animals of similar sizes and origin were kept for 
some time in filtered sea-water with uric acid, and their 
condition compared with that of control animals. In one 
experiment of this kind of animals which had been kept in 
darkness with weed for nineteen days and which had under- 
gone considerable reduction in size and in the number of their 
contained yellow-brown cells, some were put in filtered 
sea-water with uric acid, others were left with weed in 
the dark. After three days (September 9th—12th) the 
animals which had remained with weed in the dark were very 
colourless, and had lost most of their yellow-brown cells ; 
