52 PROTOCOCCOIDE^E. 



asexual reproduction takes place by zoospores (or by motionless 

 cells without cilia), which unite and form a family similar to the 

 mother-family, inside the mother-cell, or in a mucilaginous enve- 

 lope. Where sexual reproduction is found it takes place by 

 gamete-conjugation. The principal genera are : Pediastrum (Fig. 

 50), Cwlastrum, Hydrodictyon (Fig. 51). 



The coanobiuin of Hydrodictyon reticulatum (Water-net) is 

 formed of a large number of cells which are cylindrical, and 

 attached to one another by the ends (Fig. 51). The asexual 

 reproduction takes place by zoospores, which are formed in large 

 numbers (7,000-20,000) in each mother-cell, within which they 

 move about for a time, and then come to rest and arrange them- 

 selves into a new net (Fig. 51 A) which is set free by the dissolu- 

 tion of the wall of the mother-cell, grows, and becomes a new 



B 



FIG. 50. Pediastrum agperwm. FIG. 51. Hydrodictyon reticulatum. 



A A cell where the zoospores are on the 

 point of arranging themselves to form a 

 net. B A cell with gametes svyarming out. 



coenobium. The sexual reproduction takes place by gamete-conju- 

 gation. The gametes are formed in the same manner as the zoo- 

 spores, but in larger numbers (30,000-100,000), and swarm out of 

 the mother-cell (Fig. 51 B). The zygote forms, on germination, 

 2-5 large zoospores, each with one or two cilia, these generally 

 swarm about for a time, and after a period of rest become irregular 

 thorny bodies (polyhedra) ; their contents again divide into zoo- 

 spores, the thorny external coating of the polyhedra is cast off, 

 and the zoospores, surrounded by the dilated internal coating, 

 unite to form a small family, which produces several others in the 

 manner described. 



