COLONIAL PROTOZOA 



197 



or become a gamete at the time of sexual reproduction, and is self- 

 sustaining in its metabolism and irritability. The colony may have 

 a definite shape and size and move as a whole in locomotion, but 

 otherwise its cells are as independent as those of non-colonial 

 species. Their colonial organization is only the remaining together 

 of similar cells to form a mass of characteristic size and appearance. 

 Some coordination may exist, as when the colony swims in a 

 particular direction or contracts as a whole; but the colony is not 

 a many-celled organism in the true sense, because there is no 

 division of labor, as in the metazoan, where there are different kinds 

 of cells and corresponding specializations in function. 



Colonial Mastigophora. — The comparison of unicellular with 

 multicellular organisms may ho pursued through certain of the 



Fui. 104. — Chlamydomonas, a non-colonial protozoan, and two simple 



colonial types. 



A, Chlamydomonas. B and B', two views of Gonium, sociale, a colony with only four 

 cells. C and C, two views of Gonium ptclorale, a colony with sixteen cells. 



plant-like Mastigophora, particularly the family Volvocidrr and 

 closely related forms. The members of the genus Chlamydomonas 

 (Fig. 104 A) are simple non-colonial protozoa consisting of a single 

 spherical or oval cell with two flagella, a red pigment spot like that 

 of Euglena, a prominent chromatophore, two contractile vacuoles, 

 and a cell wall. Reproduction is effected by binary fission with 

 immediate separation of the two individuals thus formed. Con- 

 jugation takes place by the permanent union of isogametes. 

 Chlamydomonas, therefore, resembles Type 3 of the series shown in 

 Fig. 110, p. 215. 



