OF THE EOTATOEIA. 651 



internal organs whilst the creatui'e is alive and the viscera fulfilling their 

 functions, — a circumstance that has even made these creatures the favomites 

 of the microscopic observer. 



Modes of Obsekving the Rotatokia. — The magnifying powers most useful 

 in the examination of the Rotatoria are those varying from 200 to 400 linear. 

 For watching their general habits, an object-glass of a half- inch focus, 

 which with an eye-piece gi\^ng a power of about 70, is ample ; but for ex- 

 amining their internal organization, one of about 300, having an object-glass 

 of from one-third to one-sixth of an inch, is the most useful ; for the special 

 examination of more minute structural details, still higher powers are occa- 

 sionally, but not frequently, needed. We have already remarked that, from 

 the transparency of their bodies, the Rotatoria can be watched with much 

 ease, theii' internal organs being distinctly visible ; and, as these latter are 

 often equally transparent with the general integument, their contents, and 

 the fimctions they perform, can be investigated with httle difficulty. When 

 theii^ general habits are subjects of investigation, it is obvious they must be 

 allowed much of the freedom enjoj^ed in their natural condition. Por this 

 piu-pose they may be introduced into a small phial of thin white glass with 

 a long narrow strip of similar material in its interior ; the latter being so 

 fixed as to be nearer one side of the phial than the other. A blade of grass 

 or one or two stalks of hay may now be introduced between the strip of glass 

 and the proximate side of the bottle ; these will attract the animalcules and 

 bring them within the range of the magnifying power. If the phial be now 

 filled with water containing the Rotatoria, they will soon find their way to 

 the vegetable matter, especially if the bottle stands for awhile in the sun, 

 with the side to which the plant is affixed turned to the light. The whole 

 may now be placed under the microscope and readily examined through the 

 lower magnif}ing powers. To some extent, the same object may be more 

 readily attained by merely transferring small fragments of the half-decayed 

 vegetation floating in the water containing the animalcules, along with a 

 drop or two of the water itself, to a glass slide, covering it over with a piece 

 of thin microscopic glass. But in this case the movements of the creatures 

 are less free, especially if they happen to be of the larger Idnds, such as the 

 FJoscularke. These are often chary of emerging from their protecting cases 

 imless the coast be clear of all impediments. But the freedom of motion, so 

 important to the accurate observation of their habits, wholly prevents the 

 examination of their internal structure. Their perpetual gjTation renders it 

 impossible to trace either the forms or relative position of the viscera ; con- 

 sequently they must be controlled. This may partly be accomplished by 

 introducing them between the glasses already recommended without the 

 intervention of any vegetable or other foreig-n substance. In this case care 

 must be taken to adjust the relations between the size of the thin glass 

 covering and that of the drop of water. If the former be large and the latter 

 small, the chances are in favour of the aninjalcules being cnished. If these 

 conditions are reversed, their motions will not be sufficiently restrained, 

 neither can the water be preserved fi'om disturbance and vibration. Hence 

 care in hitting the medium of these conditions is essential. The smaller the 

 drop of water the thinner will be the fluid film when the protecting glass is 

 placed upon it, and the more effectually wiU the vagrant habits of the 

 creatures be controlled. Sometimes it becomes necessary to rupture the 

 animals by further compression, even whilst under examination. The diffi- 

 culty is to accomplish this without forcing them out of the field of the instru- 

 ment. It may be accomplished by means of a common sewing needle fitted 

 into a handle, by which pressure may be applied gently but firmly to the 



