388 W. J. CROZIER 



The second influence has, I think, been in the past over rated. 

 Microscopic examination shows that deUcate bryozoan skeletons, 

 sometimes in pieces 5 mm. x 2 mm., pass through the intestine 

 apparently unscathed, as do also bits of echinoid spines 5-6 mm. 

 in length. There is no detectable increase in the amount of 

 finely ground material in the last centimeter of the intestine as 

 compared with that in the oesophagus. I made a number of tests 

 in which the contents of the oesophagus, or of the buccal chamber, 

 were compared with those of the last portion of the intestine, by 

 suspending the mud in tall cylinders of water; these cylinders 

 were shaken well, and the contents allowed to sediment. The 

 proportion of fine material to that of coarser grade was in all cases 

 the same. When opening the intestine of a Stichopus, one gets 

 the impression that the contents are more finely divided than 

 are those of the oesophagus, because most of the smaller par- 

 ticles are on the outside of the densely compacted mass. Never- 

 theless, some grinding may take place, but this factor seems to be 

 relatively unimportant. 



The partial solution of calcareous fragments may be of greater 

 significance. The yellow fluid contained in the stomach of an 

 'empty' Stichopus gives with indicators an apparent acidity of 

 Ph = 5.0-6.5. Fluid obtained by centrifuging the stomach 

 contents of animals engaged in feeding showed acidities varying 

 from 4.8 to 5.5, the latter being most common. There seems 

 to be an active secretion of acid at the time of feeding. This 

 acidity is adequate to dissolve some calcium carbonate, and is in 

 fact greater than that of the rain water which forms stalactites in 

 the limestone caves at Bermuda; the freshly fallen rain water is 

 at about Ph = 6.0, while that caught directly as it dripped from 

 the tip of stalactites in several caves which I investigated was 

 at about Ph = 7.9-8.0. This seems to be the most important 

 influence which Stichopus exerts, geologically; namely, the solu- 

 tion of a small amount of calcium carbonate, which is probably 

 soon precipitated again when the intestinal contents are ejected 

 into an alkaline sea water (Ph = 8.1-8.2). In this way these 

 holothurians may have played a not inconsiderable part in the 

 excavation of lagoons, and in the formation of muddy deposits, — • 



