138 



SPECIES AND THEIR ORIGIN 



from Z. arbuscula in the general form of the colony. The 

 main stem is continued to the extreme distal end of the 

 colony and terminates in a zooid; from it branches are 

 given off right and left, and on these the remaining zooids 

 are borne. To use Mr. Saville Kent's comparison, Z. arbus- 



FiG. 27. — Species of Zoothamnium. A, Z. alterttans. B, Z. 

 dtchotonium. c, Z. simplex, D, Z. affine. E, Z. nutans. (After 

 Saville Kent.) 



cula may be compared to a standard fruit tree, Z. alternans 

 to an espalier. In this species also the colony is dimorphic. 



Z. dichotomuvi (Fig. 27, b) is also dimorphic and presents a 

 third mode of branching. The main stem divides into two, 

 and each of the secondary branches does the same, so that 

 a repeatedly forking stem is produced. The branching of 

 this species is said to be dichotomous, while that of Z. alter- 

 nans is monopodial, and that of Z. arbuscula umbellate. 



Another mode of aggregation of the zooids is found in Z. 

 simplex (Y'\g. 27, c), in which the stem is unbranched and 



