158 Dr. M. Verworn's Biological Studies of Protista. 



from the water the material, the sand, the Diatomeee, or what- 

 ever it may be. If they then proceed to divide, this and 

 the foundation of the new shell takes place in the same way 

 as in the before-mentioned Monothalamia " [Euglypha^ Quad- 

 riila, Cyphoderia, &c.). 



I now endeavoured in the first place to observe directly the 

 act of inception of sand. For this purpose it was advanta- 

 geous to furnish the Protista with as much shell- material as 

 possible, in order to give them abundant opportunities of 

 taking it into them. In order that it might be readily recog- 

 nizable and distinguishable from other materials in my inves- 

 tigations I did not employ sand-grains, but finely-powdered 

 dark blue glass, the smallest fragments of which certainly 

 appeared very clear, although in contrast with the surroundings 

 they always showed a faint bluish tinge. In subsequent 

 experiments I used perfectly black glass, the finest particles 

 of which appeared olive-coloured. After putting a suitable 

 quantity of blue glass-fragments into a watch-glass, I intro- 

 duced several Difflugice with water-plants. In a sliort time 

 the Protista began to extend pseudopodia and to creep about 

 among the powdered glass. They often touched the glass- 

 fragments with their pseudopodia, but pushed them away and 

 crept past without incepting them. I observed them for a 

 very long time and repeatedly ; but at first I did not succeed 

 in observing any inception of the glass-granules. Then it 

 chanced that a clumsy Cypris came near a Difflugia and 

 pushed roughly against its pseudopodia. In a few seconds 

 the surface of the widely extended pseudopodia became 

 wrinkled and knobbed, and some glass-granules remained 

 adhering to them, which were then gradually retracted, 

 together with the pseudopodia, completely into the interior of 

 the shell. This, therefore, was the mode in which the incep- 

 tion of shell-material takes place; by mechanical irritation 

 a reflex contraction of the pseudopodia is produced, and com- 

 bined therewith a secretion of a sticky coat on their surface, 

 which enables the glass-granules to adhere to the pseudopodia 

 and to be drawn in with them. By a series of experiments, 

 which were repeated upon other species of Diffiugia, I then 

 ascertained the great regularity of this process. When a 

 Difflugia had extended its pseudopodia to a great length and 

 was creeping about between the glass plates, it was irritated 

 with a sharp needle. The same eftect was then produced 

 with great exactness ; the pseudopodia became tubercular, 

 and while previously no glass-granules were adherent to them, 

 these now clung firmly and were slowly retracted into the 

 shell. Specimens of which the shells were partially removed 



