198 



PROBLEMS RELATED TO SINGLE-CELLED ANIMALS 



A very simple type of colony is seen in Gonium sociale (Fig. 

 104 B; and cf. Type 4, Fig. 110), one of the Volvocidce. This con- 

 sists of four cells arranged in a single layer and embedded in a 

 gelatinous plate. Each cell has two flagella, a pigment spot, 

 a chromatophore, and a contractile vacuole with one pyrenoid. 

 Locomotion is by a revolution which shows first the surface and 

 then the edge of the colony. The cells are physiologically self- 

 sustaining, like those of non-colonial protozoa, since each can 

 produce a new colony if they are separated. The colonies are 

 produced asexually by division of the cells to form four daughter 

 colonies, which separate, thus ending the existence of the parent 

 colony as such ; or sexually by separation of the cells of the colony 

 to form isogametes {cf. p. 174). In the related species, Gonium 

 pectorale (Fig. 104 C), there are sixteen cells in the colony, but 

 otherwise the structure and life cycle resemble those of G. sociale. 



Pandorina morum (Fig. 105; and Type 4, Fig. 110), which is 

 slightly more specialized than Gonium, consists of eight or sixteen 



Fig. 105. — Pandorina morum, a colonial protozoan. 



A, fully developed colony composed of sixteen flagellated cells. B, formation of daughter 

 colonies by division of each cell to form sixteen. B', daughter colony free of parent colony. 

 C, formation of microgametes. d to g, union of microgamete (d) with macrogamete (d') to 

 form zygote (g). (After Oltmanns.) 



cells, rarely thirty-two, packed together in an oval mass and sur- 

 rounded by a common envelope. Each cell has two flagella, a 

 pigment spot, a chlorophyll body and a contractile vacuole. The 

 colony swims as a whole, but otherwise its cells are physiologically 

 independent. When fully grown, each cell divides to form minia- 

 ture colonies of sixteen cells, each of which breaks through the 

 envelope of the parent colony and grows to full size, repeating the 



