242 FRESH-WATER BIOLOGY 



during the process, new structures such as mouth parts and con- 

 tractile vacuoles usually being formed as division goes on. The 

 production of swarm spores is common among the flagellates, occur- 

 ring either in the free swimming or encysted condition and develop- 

 ing into the adult either directly or after the fusion of two of them 

 has taken place. Swarm spores are produced in a few species of 

 cihates during encystment. 



Conjugation occurs in both Mastigophora and Infusoria. In 

 some cases the fusion is permanent; in others it is temporary, the 

 cells separating after an interchange of micronuclear material. 

 Conjugation may often be followed by either encystment or the 

 production of swarm spores, or both. Gametes of unequal size are 

 frequently produced, in some cases union between two small gametes 

 taking place, in others a large and a small one uniting. Among 

 Vorticellidae there is a complete fusion of the free-swimming micro- 

 gamete with the fixed macrogamete. In some of the more compli- 

 cated flagellates, as Volvox, phenomena closely resembling sexual 

 reproduction occur; sex ceUs are differentiated from somatic 

 cells, ova and sperm are developed, and new colonies are produced 

 as a result of fertiUzation. Encystment occurs in Mastigophora 

 and Infusoria as in Sarcodina, the condition sometimes being pre- 

 ceded by conjugation or followed by the formation of swarm 

 spores. 



In general, methods of collecting, studying and preserving 

 Sarcodina may be employed for Mastigophora and Infusoria. 

 However, these latter are often free-swimming, swift-moving forms, 

 and before any satisfactory study of them can be made their move- 

 ments must be retarded. An aqueous solution of gelatin will check 

 the movements without killing the animals if a solution of the right 

 consistency is used and this may be obtained by trial. Egg albumen 

 may be substituted for gelatin. A drop of very dilute methyl 

 alcohol added to the water containing Protozoa will usually nar- 

 cotize them. Evaporation of water from under the cover glass will 

 graduaUy retard their movements but the larger forms wiU soon 

 be crushed by the weight of the cover unless the latter is supported 

 by wax feet, bits of paper, or very thin glass. Fine capillary tubes 

 broken into short pieces make useful rollers on which the cover 



