PHYLUM PROTOZOA 



79 



In Pandorina (Fig. 56) the cells of the colony escape from the 

 common gelatinous envelope (C) and copulate in pairs (D, E), 

 forming a zygote (F, G), which, after a period of rest (#), divides 

 and forms a new colony (K). In some cases the copulating cells 

 are of two sizes, union always taking place between a large cell or 

 megagamete and a small cell or microgamete. In Volvox (Fig. 58) 

 this dimorphism reaches its extreme, producing a condition of things 

 closely resembling what we find in the higher animals. Certain of 

 the zooids enlarge and form megagametes (B> ovy.), others divide 

 repeatedly and give rise to groups of microgametes (B, spy. E, F), 

 each in the form of an elongated yellow body with a red pigment- 

 spot and two flagella. These are liberated, swim freely, and fuse 



n 



IMonosiga. 



2.Salpingoe ca. 



S.Polyoeca. 4.Proferospongia. 



FIG. 59. Various forms of Choanoflagellata . 2b illustrates longitudinal fission ; 2c, the pro- 

 duction of flagellulse ; c. collar ; c. vac. contractile vacuole ; fl. flagellum ; 1. lorica ; nu. 

 nucleus ; s. stalk. (After Saville Kent.) 



with the stationary megagamete (G), producing a zygote (H), 

 which, after a period of rest, divides and reproduces the colony. 

 It is obvious that the megagamete corresponds with the ovum of 

 the higher animals, the microgamete with the sperm, and the zygote 

 with the oosperm or impregnated egg. 



ORDER 2. CHOANOFLAGELLATA. 



General Structure. The members of this group are distin- 

 guished by the presence of a vase-like prolongation of the proto- 

 plasm > sometimes double, called the collar (Fig. 59, 1, c.), surrounding 



