PRIMITIVE MULTICELLULAR ANIMALS 



side of the plate, the cells are independent in their functions, and hence each 

 is physiologically balanced. This is indicated by their behavior in reproduc- 

 tion: asexually, each of the cells may divide repeatedly to form a small 

 daughter colony, and these then separate and grow to the size of the parent 

 colony. Se.xual reproduction is brought about when the mdividuals of the 

 colony separate as isogametes (gametes without differences in size) and unite 

 in pairs to form zygotes. From the zygotes new daughter colonies arise by cell 

 division, without separation of the daughter cells. 



Pandorina colonies consist of 8, 16, or 32 individual cells resembling 

 Chlamydomonas, arranged in an oval mass (Fig. 9.1). Reproduction may be 

 asexual, as in Gomum, when each individual divides repeatedly to form a 

 daughter colony. Sexual reproduction also occurs, involving either the fusion 

 of isogametes (isogamy) or syngamy between different-sized gametes (aniso- 

 gametes); a transition to anisogamy is indicated here. 



Eudorina is a colony of 8, 16, 32, or even 64 individuals (Fig. 9.2). Re- 

 production occurs as in Pandorina, except that in sexual reproduction an- 

 isogamy is the rule, the two kinds of gametes being produced consistently 

 in different colonies. 



Pleodonna, sometimes considered a variety of Eudonna, exists in two forms 

 showing interesting differences. Whereas in the colonial species just de- 

 scribed, all the cells of the colony are capable of reproduction, in Pleodonna 

 illmoisensis a group of 4 individuals at one pole of the spherical, 32-cell colony 

 are smaller and are incapable either of asexual reproduction or of forming 

 gametes (Fig. 9.2). Only 28 of the cells may thus be said to retain the 

 physiological balance characteristic of all the cells in a Pandorina colony, 

 for example. In another species, Pleodonna calijornica, only half the 64 or 128 

 cells retain the ability to reproduce; the other half have become, in effect, 

 somatic cells. 



Fig. 9.2. Further development of colonial plant-like flagellates. A, Eudorina elegans, a hol- 

 low, ovoid colony of flagellated individuals; the flagella have been omitted from this figure, 

 /i, Pleodonna illmoisensis, in asexual reproduction. The 4 "somatic cells," or sterile indi- 

 viduals, remain undivided, whereas each of the other 28 individuals ("germ cells") produces a 

 daughter colony. 



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