CELL AGGREGATION AND DIVISION OF LABOR 87 



colony. It is composed of groups of flagellated cells borne on the tips of 

 a branched stalk. 



A series of dendritic colonies beautifully graded in complexity is 

 exhibited by species of Vorticella, Epistylis, Carchesium, and Zootham- 

 nium. Individuals belonging to species of Vorticella are unicellular and 

 never form organic colonies. Each individual is provided with a contrac- 

 tile stalk. Cell division is followed by the complete separation of one 

 or both cells from the old stalk. The free cells swim about and finally 

 each attaches itself to some object by its aboral end and then produces a, 

 new stalk. The species of the other genera named above occur as more 

 or less complex tree-like colonies. In some species of Epistylis there 



Fig. 55. — Anthophysa vegetans Miiller. Spheroidal colonies arranged on a branching 

 stalk, thus combining two types of colonies. The stalk is made up of many fibers. (After 

 Kent.) 



may be as few as two cells on a non-contractile stalk, but in E. flavicans 

 there may be hundreds of individuals and many primary stalks with 

 their branches imbedded in a mass of jelly of the size of a walnut or even 

 of a baseball. Such a colony is a combination of the dendritic and 

 spheroid types. 



Spheroid colonies are somewhat spherical or ellipsoidal in form. 

 Their cells usually form a superficial layer in a gelatinous mass (Fig. 56). 

 Stalks may connect the cells with a common center, as in Synura or in 

 some species of Uroglena, while in other genera there may be no stalks. 



A beautiful series of spheroid aggregations illustrating development 

 in complexity is found in the Volvocales. The genera comprising this 

 series are Gonium, Pandorina, Eudorina, Pleodorina, and Volvox. 

 The members of the series probably all took their origin from a form like 

 Chlamydomonas, whose single cell is furnished with a nucleus, chloro- 

 plast, pyrenoid, stigma, vacuoles, two flagella, and a cell wall (Fig. 57). 



