126 



PROTOZOA 



as in the Algal Conjugatae. Zoochlorella is symbiotic in green 

 Ciliata (pp. 153 f., 158), Sponges (p. 175), Hydra (p. 256), and 

 Turbellaria (Vol. II. p. 43). 



Of the Volvocidae, Volvox (Fig. 44) is the largest and most 

 conspicuous genus. Its colony forms a globe the size of a pin's 

 head, floating on the surface of ponds, drains, or even puddles or 

 water-barrels freely open to the light. It has what may be 

 be called a skeleton of gelatinous matter, 1 condensed towards the 

 surface into a denser layer in which the minute cells are 

 scattered. These have each an eye-spot, a contractile vacuole, 



10 ix 



C.TU- 



c.v/ 



Fig. 43. Sphaerelln 

 pluvialis. A, motile 

 stage ; B, resting 

 stage ; C, D, two 

 modes of fission ; E, 

 Sphaerella lacustn's, 

 motile stage, chr, 

 Chromatophores ; 

 c.vac, contractile 

 vacuole ; c.w, ccll- 

 wall ; fl, rlagella ; 

 nu, nucleus; mi', nu- 

 cleolus ; pyr, pyre- 

 noids. (From Par- 

 ker's Biology.) 



and two flagella, by the combined action of which the colony is 

 propelled. Delicate boundary lines in the colonial wall mark 

 out the proper investment of each cell. The cells give off 

 delicate plasmic threads which meet those of their neighbours, 

 and form a bond between them. In that half of the hemi- 

 sphere which is posterior in swimming, a few (five to eight) 

 larger cells (" macrogonidia " of older writers) are evenly distri- 

 buted, protruding as they increase in size into the central jelly. 

 These as they grow segment to form a new colony. The 

 divisions are only in two planes at right angles, so that the 

 young colony is at first a plate, but as the cells multiply the 

 1 Within which is often harboured the Rotifer, Proales parasita, Vol. II. p. 227. 



