CILIATA 



151 



but its old nuclear apparatus is replaced by the fusion-nucleus. 

 This new nucleus undergoes repeated fissions ; its offspring enlarge 

 unequally, the larger being differentiated as mega-, the smaller 

 as micro-nuclei. The mates now separate (Fig. 52, F, G), and 

 by the first (or subsequent) fission of each, the new mega- and 

 micro-nuclei are distributed to the offspring. Colpidium colpoda 



Tnic. ffT 



-^mm 



c.vac 



Fig. 55. — Paramecium caudatmn (Aspirotrichaceae). A, The living animal from the 

 ventral aspect ; B, the same in optical section, the arrows show the course taken 

 by food-particles, buc.gr. Buccal groove; coii., cortex; cm, cuticle; coac, con- 

 tractile vacuole ; f.vac, food vacuole ; gul, gullet ; ined, medulla ; '}nth, mouth ;, 

 nu, meganucleus ; pa.nu, mioronucleus ; trch, trichocysts discharged. (From 

 Parker's Biology.) 



offers the simplest case, on which we have founded our diagram 

 showing the nuclear relations. During conjugation the oral 

 apparatus often atrophies, and is regenerated ; and in some cases 

 the pellicle and ciliary apparatus are also " made over." 



In the Peritrichaceae the mates are unequal; the larger is 

 the normal cell, and is fixed ; the smaller, mobile, is derived from 

 an ordinary individual by brood -divisions, which only occur 

 under the conditions that induce conjugation (Fig. 60). Here, 

 though the two pairs of nuclei are formed, it is only the migratory 



