210 Dr Gairdner's Analysis of 



anterior part of the animalcule's bod}', and indicated by the 

 movements of the particles of indigo in a state of very minute 

 division in different directions, and generally all converging to- 

 wards the orifice or mouth of the animal, situated, not in the 

 centre of the crown of cilije, but between the two rows of these 

 organs which exist concentric to one another. The attention is 

 no sooner excited by this most singular and beautiful pheno- 

 menon, when presently the body of the animal, which had been 

 quite transparent, and bearing much resemblance in aspect to 

 some of the marine Rhizostomas, becomes dotted with a number 

 of distinctly circumscribed circular spots, of a dark blue colour, 

 exactly corresponding to that of the moving particles of indigo *. 

 In some species, particularly those which are provided with an 

 annular contraction or neck (such as the Rotifer vulgaris)., se- 

 parating the head from the body, the indigo particles can be 

 traced in a continuous line in their progress from the mouth to 

 these internal cavities. 



It is requisite in these experiments to employ colouring mat- 

 ter which does not chemically combine with water, but is only 

 diffused in a state of very minute division. Indigo, carmine, 

 and sap green, are three substances which answer very well the 

 necessary conditions, and are easily recognised by the micro- 

 scope. But whatever substance is used, we must be very parti- 

 cular that it contains no lead, an impurity which very frequent- 

 ly enters into the colours of commerce. 



The microscope which Dr Ehrenberg has used in all his in- 

 vestigations is one constructed by Chevalier of Paris ; it pos- 

 sesses a power of 800. In very few cases, however, is it ne- 

 cessary to use this high power, and only to demonstrate the 

 existence of an internal cavity in those species which do not ex- 

 ceed from 1 joo ^^ 2 0^0 of ^ li"6 '" diameter, such as the Mo- 

 nas termo, atomus, and lens, and which almost elude the power 

 even of so powerful an instrument. In almost all cases, a power 

 of from 300 to 400 is sufficient ; and Ur Ehrenberg has made 

 all his observations and drawings of the structure of the Hyda- 

 tina senta with a power of 380. 



• It is as well, however, before applying any coloured solution to the drop 

 of fluid under the field of the microscope, to take a general survey of the 

 species which ve may exjiect to find in the portion under examination. 



