THE THALLOPHYTES D330 
The organisms of this order are sometimes classified 
with the Flagellates, a group having characteristics partly like » 
plants and partly like animals and represented by Euglena and 
many other unicellular, aquatic forms possessing flagella or 
cilia. 
_ Volvox globator, a typical representative of this order, is found 
in fresh water pools as a tiny, hollow, spherical, green colony 
about 19 to 19 of aninchin diameter. When examined under 
the microscope (Fig. 246), it is found to consist of hundreds of 
green, more or less spherical cells, united by fine strands of 
cytoplasm (protoplasmic bridges), the whole being enveloped 
by a gelatinous sheath. The peripheral cells are provided with 
cilia, in order that the colony may rotate and roll through the 
water. In a young colony, all of the cells are alike, each con- 
sisting of a mucilaginous-like cell-wall enclosing cytoplasm, a 
nucleus, a chloroplastid and often a red pigment spot. Ina 
mature colony, however, throughout the greater part of its 
existence, two kinds of cells may be discerned: small, sterile, 
vegetative cells that do not divide and from 10 to 12 larger 
vegetative ones that divide to form new colonies. The latter 
slip inward below the level of the smaller cells and through 
repeated divisions form a number of ciliated cells joined by cyto- 
plasmic threads, which in reality is a miniature colony. This 
then escapes to the exterior through the rupturing of the gelati- 
nous wall of the old colony. 
During autumn of the year, certain of the ordinary cells 
undergo differentiation, some to form sperm cells, others, egg- 
cells. When about three times the size of the ordinary sterile 
cells, the sperm cells divide repeatedly to form a cluster of 
elongated secondary cells [Fig. 246 (1a, a® and 5)] each of which 
contains an orange pigment, a red corpuscle, has an elongated 
beak and bears a pair of flagella (lash-like processes). The 
cluster in time separates into motile antherozoids [Fig. 246 
(6, 7)] which finally escape into the cavity of the volvox sphere 
through rupture of the investing wall. The flask-shaped egg cells 
(16, 6) increase greatly in size without dividing. Each shows 
vacuoles, then becomes filled with a dark green pigment, becomes 
spherical and acquires a gelatinous envelope. It then passes 
