652 SYSTEMATIC HISTORY OF THE I>^FrSORIA. 



thin glass. But this object can be still better attained by means of one of 

 the compressoria provided by opticians : the pressure being eiFected with a 

 fine screw, the movement can be regulated \\ith the utmost nicety. Thus 

 the animal can be merely fixed in its position, whilst its Tital functions pro- 

 ceed without interruption. On increasing the pressure, we obtain increased 

 transparency by reducing the thickness of the animal; and on carrying the 

 motion still further, we can rupture the integument, when the viscera become 

 detached and discharged through the fissure. Thus their minute organization 

 can be more accurately ascertained than when retained in situ. In some 

 cases the forms of the various viscera can be readily ascertained from the 

 different hues which characterize them, but in the majority of Rotatoria this 

 guide fails us. Consequently observers have long adopted a plan of feeding 

 the animals with brightly- coloured pigments, such as cannine and indigo, 

 which many of the creatures consume with avidity : a very small quantity of 

 the colour should be rubbed up with a httle water, as if for artistic pui'poses. 

 If the live -box be used in examining the creatures, with the water contain- 

 ing the animalcules a little of this colour must be mixed prior to the cover 

 being placed upon them. But when a common glass slide is employed, it 

 generally suffices to dip a camel's-hair pencil iato the diluted pigment and 

 apply it to the edge of the thin glass. The colour usually flows between the 

 g-lasses, and difiuses itself thi^ough the water sufficiently to answer every 

 purpose. Two objects are now attained. The minute coloui'ed particles are 

 thrown into active motion by the ciliary movements of the trochal wreaths, 

 beautifully demonstrating the force and direction of the aqueous vortices set 

 up by the animalcule ; and by noting the direction taken by such of the 

 particles as are swallowed, the position of the mouth and oesophageal canal 

 can be traced. These particles usually accumulate in the stomach, distend- 

 ing its parietes ; and as the bright colour of the pigment contrasts strongly 

 with the transparent walls of the viscus, its size, form, and position, as well 

 as the structure of its walls, can be readily made out. By prolonging the 

 observation, the same agent enables us to ascertain the dii'ection of the 

 intestine, anus, and cloaca, — since, when the stomach becomes inconveniently 

 full, the creature usually everts the cloaca, brings the anal oiifice into contact 

 with the suiTounding fluid, and suddenly empties the stomach or bowel of its 

 contents. 



There are practical disadvantages attending the use of carmine and iadigo, 

 some of which Mr. A\Tiite appears to have overcome by substituting the red 

 pigment which lines the cornea of the eye of the common house-fly (Microsc. 

 Journ. ii. p. 282). By means of a finely-pointed knife, or sharp-edged needle, 

 the large cornea can easily be detached from the head of the insect ; whilst 

 a small, stiff* camel-haii', or (still better) a small sable pencil suffices to wash 

 the pigment out of the internal concavity of the detached cornea. 



It is occasionally desirable to examine the animals by reflected instead of 

 transmitted light, in order that their true colours may be exhibited, as 

 Mr. Gosse has pointed out in the instance of Philodina citrina. 



Localities foe Eotatoria. — These are exceedingly diversified, vaiying 

 from the wide ocean to the dried-up sediment of the water-spout. There 

 are few cii'cumstances under which water exists, in which Eotatoria may not 

 be found, though they disUlie it when its contents are undergoing decom- 

 position. Consequently, though they occiu' in all vegetable infusions, they 

 are only to be found when the first stage of decomposition has passed away, 

 and they usually disappear again when the water becomes putrid and ofl'en- 

 sive. After the Monadina, Paramccia, and other smaller Infusoria have run 

 their course, and in large mea.sure disappeared, the Eotatoria occupy their 



