PARAMECIUM. 



91 



The surface is covered with cilia, in regular longitudinal 

 rows ; these serve both for locomotion and for driving 

 food particles towards the mouth. Among the cilia there 

 are small cavities in the cortex, in which lie fine protrusible 

 threads (" trichocysts "). These, though parts of a cell, 



FiG. 38. — Conjugation of Paramecium aurelia — four 

 stages. — After Maupas. 



1. Shows macronucleus (N) and two micronuclei (n) in each of 



the two conjugates. 

 ■t. Shows breaking up of macronucleus, and multiplication of 



micronuclei to eight. 



3. Shows the fertilisation in progress ; the macronucleus is 



vanishing. 



4. Shows a single (fertilised) micronucleus in each conjugate. 



suggest the thread cells of Ccelentera, and are probably of 

 the nature of weapons. The cortical layer is contractile, and 

 is distinctly fibrillated. In the substance of the cell lie two 

 nuclei, the smaller "micronucleus" lying by the side of the 

 larger " macronucleus." Food vacuoles occur as in the 



Fig. 39. — Diagrammatic expression of process 



of conjugation in Paramecium aurelia. -■ -■ -'- -.-'!-'- 



— After Maupas. 



A. The two micronuclei enlarge. 



B. Each divides into two. G 



C. Eight micronuclei result. 

 I). Seven disappear ; one (darkened) divides into two. F 



E. An interchange and fusion occurs, and the con- """' 



jugates separate. ..;. 



F. The fertilised micronucleus divides into two. p 



G. Each conjugate begins to divide, the micronucleus — 



of each half dividing into two, one of which f 



becomes the macronucleus, while the others form 

 the two normal micronuclei. The top line repre- 

 sents four individuals, each with a macronucleus A 

 and two micronuclei. 



Amoeba. There are two contractile vacuoles, from which 

 fine canals radiate into the surrounding protoplasm ; these 

 discharge into the vacuole, which then bursts to the 

 exterior. 



Life history. — Growth is followed by obliquely transverse 



