THE HYDE A AND ITS ALLIES 



219 



The variety in the form of the colony possessed even by a 

 single species adds to the diversity of hydroids. 



The Ctenophora, or sea-walnuts, are a small group of 

 exclusively marine organisms which float on the surface 

 of the sea, and like most animals having this habit have 

 become clear as glass. Many of them are highly phos- 

 phorescent (Fig. 206). 



In any colony a division of labor may occur among the 

 constituent individuals, or zooids. Thus in the simplest 

 cases we have crawling zooids, 

 or stolons, and feeding zooids. 

 In Hydractinia 1 we have, in addi- 

 tion, reproductive zooids, nettling 

 zooids, and passive, thorn-like 

 zooids. Here we see how com- 

 pletely subservient the individual 

 is to the good of the community. 

 This subserviency has ruled in all 

 successful colonies of animals. 



Regeneration. Closely allied to 

 the power of budding is that of 

 reproducing a lost organ. Suppos- 

 ing the "head" (mouth and ten- 

 tacles) of a Hydra to be cut off, the 

 base will reproduce the lost head. Suppose the base to be 

 removed, the head will reproduce a new base. When a 

 Hydra is cut in two transversely, two Hydras result where 

 formerly there was only one. Even three or more Hydras 

 may arise when a Hydra has been cut into so many pieces. 

 Where other conditions are favorable to life, you can 

 hardly kill a Hydra by mutilation. A trace of this 



i Tig. 195. 



FIG. 206. Idylia, a sea- 

 walnut, seen from the 

 broad side. Half nat. size. 

 a, anal opening ; b, lateral 

 tube ; c, circular tube ; d, c, 

 /, g, h, rows of paddles. 

 After Agassiz. 



