PHYLUM PROTOZOA 



77 



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 conjugate 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. 



It should be noticed that in the more complex cases of repro- 

 duction just described we meet with a phenomenon not seen in 

 cases of binary fission, viz., development, the young organism being 

 far simpler in structure than the adult, and reaching its final form 

 by a gradual increase in complexity. 



IMonosiga. 2.Salpinooeca. 



S.Polyoeca. 4.Proterospongia. 



I.'K:. oS. Various forms of Choaiiofiagellata. 2b illustrates longitudinal fission ; 2c, the pro- 

 duction of flagellulaj ; c. collar ; c. vac, contractile vacuole ; jL flagelium ; 1. lorica ; nu. 

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



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. 58,7, c.), surrounding 

 the base of the single flagelium (fl.). The collar is contractile, and, 

 although its precise functions are not yet certainly known, there is 



