216 COMPARATIVE ANATOMY 



may be transferred, by means of a pipette, to a nutrient 

 medium. A suitable medium can be made by boiling hay in 

 water, decanting off the liquid, and allowing it to stand. The 

 infusion will soon be rendered turbid by the presence of 

 Bacteria, and as Paramecia live on Bacteria, they will feed 

 greedily and multiply with great rapidity. 



Paramecium caudatum and P. aurelia are species so similar 

 to one another that they have often been called by the same 

 name. They are relatively large, attaining a length of '3 mm., 

 so that they are visible to the naked eye as whitish specks in 

 the water when the glass containing them is held up to the 

 light. We will confine our attention chiefly to P. caudatum, 

 but the account given here will apply, as far as structure is 

 concerned, almost equally well to P. aurelia. The latter 

 species differs from the former principally in having two 

 micronuclei instead of one,* and also its posterior extremity 

 is rounded and blunt, whilst that of P. caudatum is pointed. 

 A third species Paramecium bursaria is almost as common as 

 the other two, but it is much less convenient for study, as its 

 protoplasm is rendered opaque by the presence of numerous 

 round green corpuscles, which are minute algae living in the 

 body of the infusorian and sharing its nourishment. The 

 algae, whilst they find protection in its body, are also of direct 

 benefit to their host, for they manufacture starch through the 

 agency of their chlorophyll, and from time to time their starch- 

 laden bodies are swept into the endoplasm of the infusorian, 

 are borne round in the food current, and digested. This 

 intimate union of an animal and plant is paralleled in several 

 other groups of the animal kingdom, and the phenomenon is 

 technically called Symbiosis. 



The first thing to observe in Paramecium caudatum is its 

 shape : it preserves a perfectly definite and constant form as 

 long as it is alive, never entering into an amoeboid condition, 

 nor exhibiting euglenoid or pseudopodial movements. It is, 

 however, flexible and elastic, and may be observed to bend 

 its body in passing round an obstacle, and to squeeze itself 

 through an aperture smaller than its own diameter. 



* It has, however, recently been stated that the possession of one or 

 two micronuclei does not constitute a sufficient specific distinction be- 

 tween P. caudatum and P. aurelia. 



