136 



SPECIES AND THEIR ORIGIN 



(Fig. 28, A) is found also in sea-water, and differs markedly 

 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. 28. Species of Zoothamnium. A, Z, alternans. B, Z. 

 dichotomum. c, Z. simplex. D, Z. ajfine. 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. dichotomum (Fig. 28, 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. 



