96 



PRINCIPLES OF ANIMAL BIOLOGY 



plicated. An account of the structure of tissues, organs and systems is 

 deferred to the following chapter and the functions of organs to Chapter 

 VII. 



Colony Formation in Metazoa. — Among the metazoa as among the 

 Protozoa certain methods of asexual reproduction, such as budding or 

 fission, ma}' give rise to aggregates or colonies of individuals. The term 

 colony is strictly applicable only to groups of individuals with organic 

 connections and is so used in this book. 



An understanding of the nature of colonies will proceed more easily 

 from a knowledge of similar animals in species which do not form colo- 

 nies. Especially useful for this preliminary purpose, because its near 

 relatives are mostly colonial, is the simple freshwater Hydra. Hj'-dra (Fig. 



60) reproduces by budding. As a bud in- 

 creases in size it may give rise to another 

 bud; and before the first bud becomes sepa- 

 rated from the parent three or four buds may 

 begin to grow at other points on the surface 



Fig. 01. — The hydroid, Bou- 

 yainvillea ramosa, ijortion of a 

 colony, p, polyp; mh, medusa 

 bud. {After Allman.) 



Fig. 62. — Medusa of Bou(iainvillca ramosa 

 freed from the colony, enlarged more than Fig. 

 61. (After Allman.) 



of the parent's body. The parent with its buds forms, in effect, a tem- 

 porary colony. The association does not become permanent because the 

 cells of the parent finally close the connection between the digestive cavity 

 of parent and that of each of the buds and the buds are gradual^ pinched 

 off. However, while colon}' formation never occurs in Hj^dra it is very 

 common in the phylum Ccolentcrata to which Hydra belongs, being es- 

 pecially common in the classes Hydrozoa and Anthozoa. The classical 

 examples are the hydroids, or Hydra-like colonial members of the former 

 class. 



Colonies in Hydrozoa and Corals. — Excellent examples of colony 

 formation may be found in the hydroid genera Bougainvillea, Obeha, 

 and Hydractinia. Bougainvillea ramosa (Fig. 61) forms a colony with a 



