INFUSORIA. 27 



SUB-CLASS I. MASTIGOPHORA. 



Infusoria generally of small size provided with flaydla. 



This sub-class includes forms Avhich live in putrefying infusions, 

 parasitic forms, and forms which live freely. They all have con- 

 tractile vacuoles, and some of them have an opening at the base of 

 the fiagellum for the reception of solid substance (holozoic nutrition). 

 Encystment and spore formation are very commonly found ; and 

 it has been shown by Dallinger and Drysdale* that the spores are 

 capable of resisting a temperature above boiling point. Dallinger 

 has also shown, in the case of some of the infusion forms, that 

 it is possible by very gradually raising the temperature in which 

 the animals are living during a number of successive generations to 

 produce a race for which the optimum temperature is considerably 

 above the normal killing temperature for the species. 



Conjugation is known to occur very generally, and in some cases 

 the conjugating individuals (or gametes) \ are especially differen- 

 tiated. This is notably the case in Volvox, in which the gametes 

 are of two kinds, and recall the spermatozoa and ova of the higher 

 animals. Many members of our sub-class are difficult to distinguish 

 from the swarm-spores of certain Rhizopoda and of the Myeetozoa, 

 and even from the zoospores of unicellular AJijw. It is necessary, 

 therefore, to point out that in the Mastigophora the flagellated 

 stage covers the main, if not the entire, period of the life of the 



organism. 



The nucleus is almost invariably single. 



Order 1. FLAGELLATA. : 



Mastigophora withflagella, without collar or cilia. 



This order includes holozoic, holophytic, and saprophytic forms. 

 Many of them are parasitic and many live in infusions. It is not 

 infrequent to find an amoeboid condition of the body combined with 

 the possession of the fiagellum (Mastigamceba), or to find these two 

 conditions alternating in the life -history. Many of them form 

 colonies, and an outer cuticular skeleton in the form of a cup or 

 investing membrane may be present ; and in some forms a gelatinous 

 layer is secreted. In the holozoic forms food may be taken up by 



* "Researches on the Life-history of the Monads." Monthly Hie. Journal. 

 10-13. 



t An organism which conjugates, whether specially differentiated or not, is 

 called a gamete, and the product of the conjugation is a zygote. 



t G. Klebs, " Flagellatenstudien." Z. f. w. Z., 55, 1892. 



