42 



OUTLINES OF EVOLUTIONARY BIOLOGY 



after each cell-division. In Pandorina (Fig. 10), however, which 

 is another common fresh-water organism, the individuals pro- 

 duced by fission remain together and form a solid ball, composed 

 of from sixteen to sixty-four cells, enclosed in a gelatinous 

 envelope. These little colonies swim about actively by means of 

 their flagella, which project from the surface in pairs, one pair 

 belonging to each individual. Multiplication is usually effected 

 by the division of each individual cell into sixteen, so that as 

 many daughter colonies are formed as there were cells in the 

 parent colony, and these daughter colonies finally separate 



from one another. Eudorina 

 (Fig. 40) forms slightly larger 

 colonies of a similar kind, but 

 with the component individuals 

 somewhat widely separated from 

 one another by the gelatinous 

 matrix. Finally, in Volvox 

 (Fig. 11), one of the most 

 familiar and beautiful of the 

 microscopic fresh -water organ- 

 isms, we find the individual 

 cells, each one still closely 

 resembling a HcTBmatococcus, 

 arranged side by side in the 

 gelatinous wall of a hollow 

 sphere, with their flagella pro- 

 jecting from the surface, whilo 

 daughter colonies are frequently seen swimming about freely in 

 the interior of the sphere. 



In all these colonies the gelatinous matrix or ground substance 

 is formed as a secretion by the cells which it serves to hold 

 together. It is noteworthy that the connection between the 

 individual cells is much more intimate in Volvox than it is in the 

 lower types, for they are all united together by extensions of 

 their protoplasmic bodies which give a reticulate appearance to 

 the wall of the sphere. Volvox, moreover, attains a relatively 

 large size, and there may be as many as 22,000 cells in a single 

 colony, though about half that number appears to be more 

 usual. 



We meet with another example of the formation of hollow 

 spherical colonies in the case of the beautiful Radiolarian, 



FlG. 10. Pandorina morum, X 400. 

 (From Vines' " Botany.") 



A, free-swimming colony ; B, conjugation 

 of two gametes. 



