160 The Mastigophora 



spheroid packet results when fission produces 256 or more cells; the in- 

 verted plakea remains plate-like when the number is only 16-128. 



The developing macrogamete of Volvox, early in the life of the young 

 colony, grows into a large spheroidal cell containing much stored food. 

 Microgametes enter female colonies, sometimes before the ova are fully 

 grown, and finally penetrate the ova as they approach or reach maturity. 

 After fertilization, the zygote encysts. The ectocyst may show characteristic 

 decorations (Fig. 4. 32, H-K). After disintegration of the female colony 

 the cyst sinks to the bottom, where it remains dormant until the follow- 

 ing spring. Under natural conditions, colonial forms may occur only 

 during two or three months of the year, so that the encysted zygote 

 is the predominant phase of the cycle (259). In laboratory cultures, how- 

 ever, repeated generations of asexual colonies have been obtained over a 

 period of a year or more (275). 



Order 6. Euglenida 



The Euglenidai are rather large flagellates, mostly with one or 

 two flagella. The body is generally elongated and often spindle-shaped, 

 with some degree of spiral torsion, but modifications occur in such genera 

 as PJiacus (Fig. 4. 34, I-L). The reservoir (Fig. 4. 33, A-D), or "gullet," 

 from which the flagella arise, is a characteristic feature. Flagellates as- 

 signed to two genera, Chlorachne and Ottonia, apparently lack reservoirs, 

 but Schiller's (251) descriptions do not supply conclusive evidence that 

 these are Euglenida. One or two contractile vacuoles empty into the 

 reservoir, and each flagellum is inserted in the posterior or postero-dorsal 

 wall of this cavity. The pellicle permits euglenoid movement (metaboly) 

 in many species but it may be only slightly flexible, as in Euglena acus, 

 or rather rigid in such genera as Menoidium and Phacus. As reported for 

 Euglena viridis (228), this membrane gives negative tests for cellulose, 

 but is completely digested by trypsin and presumably contains proteins. 

 According to Chadefaud (34), the pellicle (Fig. 4. 33, E) consists of a thin 

 epicuticle and a deeper and thicker cuticle. Only the epicuticle extends 

 into the reservoir. The usually noticeable spiral striations seem to be 

 cuticular ridges (34); presumably the rows of papillae in Euglena 

 spirogyra are comparable decorations. The distribution of peripheral 

 inclusions, and sometimes that of the chromatophores, may follow the 

 spiral decorations of the pellicle. In addition to the pellicle, a lorica oc- 

 curs in Ascoglena and Klebsiella (Fig. 4. 33, K, L); a shell, or test, in 

 Trachelomonas (Fig. 4. 33, J). 



Perhaps the majority of Euglenida are chlorophyll-bearing, although 

 there are many colorless species. The chromatophores range from one to 

 many and also vary in size and form (Fig. 4. 33, F-J) in different species. 

 The green color of chlorophyll is not masked by other pigments. How- 



^ The literature on Euglenida has been reviewed by Jahn (119). 



