1 88 COMPARATIVE ANATOMY 



or three days the water will swarm with Bacteria, these will give 

 way to Flagellates, and then the Paramecia contained in the 

 water will feed greedily on the Flagellates 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 algs 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 manufac- 

 ture 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 swept 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 Ssoubiosis. 



The first thing to observe in Paramecium caudatum is its 

 shape. Unlike the Protozoa which we have hitherto studied 

 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. 



The general shape of the animal is that of a spindle 

 pointed at one extremity and rounded off at the other. (Fig. 

 41, A.) It always moves with the blunt end foremost, so this 

 may at once be distinguished as the anterior, the opposite 

 pointed end as the posterior extremity. Further, it has a 



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