60 



BIOLOGY 



appears in the scum that forms on the surface, and among them 



may be seen some minute white specks, just visible to the naked 



eye, each one of which is a Parame- 

 dum; Fig. 21. 



Like the Amoeba, it is a single cell, 

 and like the Amoeba also, it is made 

 up of protoplasm consisting of an 

 outer, somewhat clear ectoplasm and 

 an inner, more granular endoplasm. 

 The Paramedum has a body which, 

 although flexible, is somewhat rigid 

 and elastic, and, unlike the Amoeba, 

 always tends to preserve a definite 

 form. It is elongated, somewhat 

 blunter at one end than the other, 

 and in its motion carries the blunt 

 end forward. The protoplasm has 

 no power of protruding pseudopodia, 

 and the animal therefore does not 

 change its shape like the Amoeba. 

 Upon one side, posterior to the 

 middle of the body, there is a groove 

 extending obliquely backward. This 

 is the oral groove (og), at the bottom 

 of which there is an opening leading 

 to a short tube which extends through 

 the ectoplasm into the endoplasm. 

 The opening is the mouth, and the 

 tube is known as the oesophagus or 

 gullet; oe. 



Locomotion. The whole of the outer 

 surface of the animal is covered with 

 numerous, fine, threadlike projections 



of protoplasm, protruding from the ectoplasm into the water. 



These are called cilia and are capable of rapid motion back 



CV 



ex 



FIG. 21. PARAMECIUM 



AURELIA 



ci, cilia; 



cv, contractile vacuole; 



ex, excreta; 



m, mouth; 



mic, micronucleus ; 



mn, macronucleus; 



mb, membranella; 



oe, 03sophagus; 



og, oral groove. 



