20 



INFUSORIA. 



whose protoplasm unites together, together with the nuclei, 

 forming one larger individual, which is again in a fit state to 

 undergo once more rapid division. This conjugation is really a 

 kind of primitive sexual reproduction, although there is, as far 

 as we can see, no difference between the conjugating individuals. 

 Infusoria (fig. 3, i., ii., and iii.) are those protozoa which, unlike 

 Amoeba, have a definite form, with an external membrane, which 

 bears either cilia or flagella. Cilia are fine, short, hair-like threads 

 of protoplasm ; flagella are long whip-like processes that pene- 

 trate the membrane of the animalcules. The Infusoria were 



Fig. 3. — Infusoria and Foraminifeba. 



i. Euglena. ii. Cercomonas iniestinalis. iii. Pdytoma, free and encysted, 

 iv. Textularia. v. Globigeritia. 



discovered at the end of the seventeenth century. The outer mem- 

 brane is in the form of a cuticle, showing a double contour. These 

 protozoa are usually to be found in liquid media, such as putrid 

 water and other fluids ; whilst many are found living in the ali- 

 mentary contents of man and other animals, as also in other parts 

 of the body. Sometimes they are so far parasitic as to cause seri- 

 ous disorders, and even death. According as to whether there 

 are numbers of cilia surrounding the infusorian or only one, two, 

 or more whip-like flagella, these protozoa are placed in two divi- 

 sions, known respectively as the Ciliata and the Flagellata. 



