114 PARAMCECIUM, STYLONYCIIIA, OXYTRICIIA less, x 



and by the contraction of the cortex consequent upon any 

 sudden irritation they are projected in the way indicated. 

 In Fig. 20 B, a few trichocysts {trcH) are shown in the ex- 

 ploded condition, i.e. with the threads protruded. Most 

 likely these bodies are weapons of offence like the very 

 similar structures (nematocysts) found in polypes (see Lesson 

 XXI. Figs. 50 and 51). 



Paramoecium multiplies by simple fission, the division of 

 the body being always preceded by the elongation and 

 subsequent division of the mega- and micronucleus (Fig. 

 20, d). Division of the meganucleus is direct, that of the 

 micronucleus indirect, i.e. takes place by mitosis. 



Conjugation also occurs, usually after multiplication by 

 fission has gone on for some time, but the details and the 

 results of the process are very different from what are found 

 to obtain in Heteromita (p. 41). Two Paramoecia come 

 into contact by their ventral faces (Fig. 21, a) and the mega- 

 nucleus {mg. nu) of each gradually breaks up into minute 

 fragments (d — g) which are either absorbed into the proto- 

 plasm or ejected. At the same time the micronucleus 

 {mi. mi) divides, by mitosis, and the process is repeated, 

 the result being that each gamete contains four micro- 

 nuclei (b). Two of these become absorbed and disappear 

 (c, mi. nu, mi. nu") ; of the remaining two one is now distin- 

 guished as the active pronucleus, the other as the stationary 

 pronucleus. Next, the active pronucleus of each gamete 

 passes into the body of the other (c) and fuses with its 

 stationary pronucleus (d) : in this way each gamete con- 

 tains a single nuclear body, the conjugation-nucleus (e), 

 formed by the union of two similar pronuclei one of 

 which is derived from another individual. It is this 

 fusion of two nuclear bodies, one from each of the con- 



