72 THE VITAL FUNCTIONS. 



which acted as poisons, instead of those which might serve 

 as food, he failed in his endeavours. Equally unsuccessful 

 were the trials made by Ehrenberg with the indigo and gum- 

 lac of commerce, which are always contaminated with a cer- 

 tain quantity of white lead, a substance highly deleterious to 

 all animals; but, at length, by employing an indigo which 

 was quite pure, he succeeded perfectly.* The moment a 

 minute particle of a highly attenuated solution of this sub- 

 stance is applied to a drop of water in which are some pe- 

 dunculated vorticellse, occupying the field of the microscope, 

 the most beautiful phenomena present themselves to the eye. 

 Currents are excited in all directions by the vibrations of 

 the cilia, situated round the mouths of these animalcules, 

 and are readily distinguished by the motions of the minute 

 particles of indigo which are carried along with them; the 

 currents generally all converging towards the orifice of the 

 mouth. Presently the body of the vorticella, which had 

 been hitherto quite transparent, becomes dotted with a num- 

 ber of distinctly circular spots, of a dark blue colour, evi- 

 dently produced by particles of indigo accumulated in those 

 situations. In some species, particularly those which have 

 a contracted part, or neck, between the head and the body, 

 as the Rotifer vulgaris, these particles can be traced in a 

 continuous line in their progress from the mouth to these 

 internal cavities. 



In this way, by the employment of colouring matters, Eh- 

 renberg succeeded in ascertaining the existence of a system of 

 digestive cavities in all the known genera of this tribe of 

 animals. There is now, therefore, no reason for admitting 

 that cuticular absorption of nutritive matter ever takes place 



* The colouring matters proper for these experiments are such as do not 

 chemically combine with water, but yet are capable of being- diffused in a 

 state of very minute division. Indigo, sap, green, and carmine, answer these 

 conditions, and being also easily recognised under the microscope, are well 

 adapted for these observations. Great care should be taken, however, that 

 the substance employed is free from all admixture of lead, or other metallic 

 impurity. 



