OLIGOTKKMA PSAMMITES. 263 



mucli sliredded ; it was only liere and tliere that I was able 

 to find adherent patches snch as that represented in the 

 figure. The strips lyiug loose in the cavity are very 

 nnmerons^ and those cut through in any section would go 

 several times round the cavity if placed end to end — a fact 

 which suggests considerable complication in the way of folds. 

 The spherical concretions are often found embedded in the 

 epithelium, as shown in fig. 24. 



De Lacaze Duthiers, Kuppfer, and Krohn made a careful 

 study of the contents of the renal sacs of the species they had 

 under observation, and agree in stating that the spherical 

 concretions gave the murexide reaction with nitric acid and 

 ammonia; but in each case the reaction appears to have been 

 faint, and their descriptions are not very convincing as to 

 the presence of uric acid. In order to test this point I dis- 

 mounted one of my slides and applied the murexide test, 

 without obtaining any trace of a red or purple colour. The 

 concretions and the surface layers of the coagulum were 

 stained yellow, which suggested the xanthroproteic reaction 

 and would seem to indicate the presence of proteid reserve 

 material. It is obviously impossible to draw conclusions from 

 chemical tests applied to sections which had previously 

 undergone prolonged treatment for staining and mounting*, 

 but it is worth recording the fact that I failed to obtain the 

 murexide reaction, for the structure and relations of this so- 

 called renal sac are more suggestive of storage of reserve 

 material than of an excretory organ, and the chemical com- 

 position of the concentricall}^ striated spherules requires 

 further investigation. In the spherules of Oligotrema the 

 darker concentric bands were stained orange-red by borax 

 carmine, and were deeply stained by hasmatoxylin. The 

 central coagulum was unaffected by these dyes, but stained 

 vividly with eosin ; the outer stratified layers stained in 

 the same manner as the spherules. Fig. 33 shows the 

 manner in which the spherules seem to be formed from the 

 external stratified layers of the coagulum contained in the 

 renal sac. 



