THE MASTIGOPHORA 



275 



Three families are recognized. The first, represented by the genus Pyra- 

 mimonas, contains primitive forms in which the body is metabolic and the 

 cellulose envelope is absent. The second family, CMamydomonididce, com- 

 prises non-colonial forms such as Chlamydomonas, Hcematococcus, etc. Nephro- 



B 



l'io. 119. Gonium pectorale: colony of sixteen individuals, each with two nagella. 

 A, In surface view ; B, in side view. N., Nuclei ; c.v., contractile vacuoles ; 

 st., stigmata. After Stein. 



selmis. referred by Senn (358) to this family, has two flagella, on which it 

 creeps like a Bodo. The third family, Volvocidce, comprises colony-forming 

 species in which the individual is similar in structure to the Chlamydomonads, 

 and the colony is composed of individuals ranging in number from four, eight, 



