CHAPTER III. 



HYDROIDEA. 



11. The Hydroidea are minute polyps, of extreme simplicity of 

 structure and delicacy of form. Though sometimes single animals, 

 swimming at large, like the Hydra, they usually constitute compound 

 zoophytes, hundreds and often thousands to a cluster. Some, as in 



Fiff. 1. 



the annexed figures,* grow in crowded tufts of thread-like stems: 

 many are much branched, and each branch is tipped with a star of 



* These figures are by J. P. Couthouy, and represent a Tubularia from Rio de Janeiro, 

 which he designates T. ornata. Figure 1 shows the natural size of the animals. 



