112 



C. M. CHILD. 



FIG. 10. Apical region 

 of stem of Obelia borealis 

 Nutting, to show sympo- 

 dial method of branching: 

 (a) Latest bud on main 

 axis; arose as a lateral bud 

 from the axis bb'. The 

 axis bb' arose as a lateral 

 bud from the axis cc', and 

 axis cc' as lateral bud from 

 axis dd'. As the stem of 

 hydranth d elongated a 

 second bud appeared and 

 produced the axis ff, and 

 from this axis a new axis 

 g is just developing. The 

 stem of hydranth c has 

 also produced a second bud 

 which has given rise to the 

 axis e. In this way the 

 lateral branches develop. ' 



the susceptibility relations indicate that 

 the medusa represents physiologically a 

 more advanced stage of development than 

 the fully developed hydranth and is there- 

 fore physiologically older and less suscept- 

 ible. 



COLONY GRADIENTS. 



The hydroid colony as a whole represents 

 a more or less orderly integration of the 

 constituent units. The general growth 

 form or arrangement of zooids is more or 

 less definite and characteristic for the 

 species, and this definiteness of form and 

 arrangement is particularly characteristic 

 of those species in which a main stem and 

 lateral branches, with perhaps secondary 

 and tertiary branches can be distinguished. 

 In these respects the hydroid colonies re- 

 semble multiaxial plants, and such plants 

 are colonial forms in the same sense as the 

 hydroids, i.e., each growing tip in the 

 plant, like each zooid in the hydroid, rep- 

 resents at least the apical region of a phy- 

 siological axis, an individual, with more or 

 less complete development of other parts. 

 Moreover, the two general types of rela- 

 tion between the different constituent axes 

 which are characteristic of plants, viz., the 

 monopodial and the sympodial forms of 

 branching, are also found in the hydroids. 

 In Pennaria and Bougainmllea and various 

 other tubularian hydroids the colony is of 

 the monopodial type, i.e., new axes arise 

 as lateral branches of a persistent main 

 axis (Fig. 9). In Obelia and various cam- 

 panularians examined the colony is sym- 

 podial in type. Here each new b*ud be- 

 comes the apical end of the apparent main 

 axis, the former apical zooid being dis- 



