THE PROTOZOA. i3g 



Something has already been said as to the repro- 

 duction of Vorticella, one of the best-known types of 

 the Infusoria. It is a bell-shaped animal on a stalk, 

 with a mouth which is protected by a ciliated flap, 

 the moving cilia of which keep up a current that 

 whirls food particles into the mouth : hence its name 

 of Vorticella (little whirlpool). Other forms, which are 

 free-swimming, have belts of cilia distributed all over 

 them. Vorf.icella may be noted as possessing striped 

 protoplasm, by means of which it appears to execute 

 its contractile movements, and similar stripes are 

 seen on the tentacle o{ Nodiluca. These are believed 

 to be the most primitive form of striated muscle, the 

 difference being, that in the latter the stripes extend 

 across the protoplasm of a series of contiguous and 

 confluent cells, instead of across a small portion of the 

 protoplasm of one cell. There are many other stalked 

 forms besides Vorticella (Sgs. 122, 123, p. 303). The 

 majority of the order are, however, free-swimming. 



The Protozoa do not always reproduce themselves in 

 the way which has been described in the case of 

 Vorticella, The organism inay divide without the 

 OQCurrence of any previous fusion of two cells ; that is, 

 it may reproduce itself asexually. It is believed, how- 

 ever, that this process cannot go on indefinitely, and 

 that, after repeated multiplication by division the 

 conjugation of two cells (fig. 21b, p. 80) eventually 

 becomes necessary as a condition of the occurrence of 

 further division. Division into two may take place, 

 or division into a number of very small cells, which in 



