POLYTOMA 



39 



that, as sometimes in Polytoma, there is a difference in size, 

 though not in any other respect, between the gametes. The syn- 

 gamy of two hke gametes, whether they be ordinary or special 

 individuals, is known as isogamy ; syngamy of unlike gametes 

 is anisogamy ; if, as in most animals, they differ not only in size 

 but also in that the larger is passive and the smaller active, 

 the process is known as oogamy. 



EUGLENA 



Euglena viridis (Fig. 14), often so 

 common in puddles as to give them a 

 green colour, is a flagellate organism of 

 a rather higher grade than Chlamydomonas 

 or Polytoma. It is a minute, spindle- 

 shaped creature, which may reach a length 

 of 0.17 mm. The front end is blunt and 

 bears one fiagellum rooted at the base of a 

 funnel-shaped pit, which is known as the 

 * gullet ' but probably never used as such. 

 The base of the fiagellum appears to ^^ 

 bifurcate but it is possible that there is 

 really a second short fiagellum since some 

 related species have two, of which one is 

 shorter than the other. On the other 

 hand, some workers think that more 

 probably two flagella have fused. The 

 electron microscope shows that the 

 fiagellum bears a row of filaments, some 

 five or six times its diameter in length. 

 There is a strong pellicle, a distinct 

 ectoplasm, and a central, spherical nucleus. 

 Band-shaped, green chloroplasts (p. 20) 

 radiate from a point in front of the nucleus, where granules of 

 the starch-like substance paramylum accumulate. Waves of 

 contraction pass along the body (Fig. 15), but contractile 

 strands (myonemes, p. 43) are lacking. There is a large 

 contractile vacuole which discharges into the expanded base 

 of the gullet. Just before the collapse a number of small 

 vacuoles appear near the old one, and these coalesce to 

 form the new vacuole (Fig. 16). The system is, therefore, similar 



Fig. 14. — Euglena viridis, 

 highly magnified. 



av.. New contractile vacuole 

 beginning to form ; cv., main 

 contractile vacuole ; chp., 

 one of the chloroplasts ; cu., 

 pellicle ; e.s., eye-spot ; ec, 

 ectoplasm ; fl., fiagellum ; 

 g., gullet ; nu., nucleus ; 

 p.g., paramylum granules ; 

 /).g'., protoplasmic mass, with 

 paramylum granules, from 

 which chloroplasts radiate ; 

 res., reservoir. 



