62 BIOLOGY 



brane known as the cuticle (cu), through which the cilia pro- 

 trude. This is ordinarily invisible and can only be seen under 

 special conditions. It is a protective covering which makes the 

 body a little more resistant than it otherwise would be. The 

 endoplasm fills the rest of the body and is very highly granular, 

 containing large numbers of food masses in various stages of di- 

 gestion. The nucleus is double, showing a large macronucleus* 

 (Fig. 21 win), and near it a small micronucleusf, mic. These 

 two bodies lie close together near the mouth and hold fairly 

 constantly their relative positions in the body of the animal. 

 Two contractile vacuoles (cv) are found in the common species 

 of Paramedum, one at each end. These vacuoles connect 

 with the different parts of the body by a number of minute 

 radiating canals, six or ten in number, which extend in all direc- 

 tions. Certain liquids are, apparently, poured into these canals 

 from the living protoplasm and through them flow into the 

 vacuoles, which increase in size until they reach a certain mag- 

 nitude and then suddenly contract and discharge their contents 

 to the exterior, probably through minute openings. The con- 

 traction of the vacuoles is fairly regular, varying in rapidity 

 with the temperature; the two vacuoles do not contract simul- 

 taneously, but alternate with each other. These organs, as in 

 the case of the Amoeba, are probably associated with the func- 

 tion of respiration and excretion. 



Assimilation and Growth. The food of the Paramedum con- 

 sists chiefly of minute bacteria. These are driven into the 

 mouth by the action of the cilia, and by the membranella in the 

 ossophagus, and then guided down the oesophagus to its inner 

 end. Here the bacteria collect in a little drop of water. The 

 oesophagus then contracts and pinches off this little drop con- 

 taining the bacteria, and thus forms what is called a food vacu- 

 ole, which enters into the general mass of the endoplasm and 

 follows the movement of the protoplasm around the body. 

 The digestive juices are secreted and gradually digest the bac- 

 * Gr. macros = large. f Gr. micros = small. 



