92 PIIYL UM PR TOZOA THE SIMPLEST ANIMALS. 



to the mouth, from which undigested residues are got rid of. 

 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 



FIG. 38. Conjugation of Paramcecinm aitrelia four 

 stages. After Maupas. 



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



the two conjugates. 



2. 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. 



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

 suggest the thread cells of Ccelentera, and are probably of 

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

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

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



FIG. 39. Diagrammatic expression of process 

 of conjugation in ParamKcium atirclia, 

 After Maupas. 



A. The two micronuclei enlarge. 



B. Each divides into two. 



C. Eight micronuclei result. 



D. Seven disappear ; one (darkened) divides into two. 



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



jugates separate. 



F. The fertilised micronucleus divides into two. 



G. Each conjugate begins to divide, the micronucleus 



of each half dividing into two, one of which 

 becomes the macronucleus, while the others form 

 the two normal micronuclei. The top line repre- 

 sents four individuals, each with a macronucleus 

 and two micronuclei. 



larger " macronucleus." Food vacuoles occur as in the 

 Amceba. 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 



o 



