PARAMGCIUM. 95 
abundant in most stagnant pools, and are just visible 
when a test-tube containing them is held between the 
eye and the light. Their food consists of small vegetable 
particles. : ; 
The form is a long oval, with the blunter end in front; 
Fic. 40.—Conjugation of Paramecium aurelia—four 
stages. —After Maupas. 
1. Shows macronucleus (/V) and two micronuclei (z) in each ot 
the two conjugates. 
z. Shows breaking up of macronucleus, and multiplication of 
micronuclei to eight. 2 
3+ Shows the fertilisation in progress; the macronucleus is 
vanishing. : 
4. Shows a single (fertilised) micronucleus in each conjugate. 
the outer portion of the cell substance is differentiated into 
a dense rind or cortex, with a delicate external cuticle, 
perforated by cilia. There is a definite opening, the so- 
called mouth, which serves for the ingestion of food 
particles ; and there is also a particular anal spot posterior 
Fic. 41.—Diagrammatic expression of process 
of ‘conjugation in Paramecium aurelia. 
—After Maupas. 
A. The two micronuclei enlarge. 
B. Each divides into two. 
C. Eight micronuclei are formed. 
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. 
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. 
