HYDRA FUSCA 

 \ 



149 



colony made up of at least five different types of members or 

 zooids. Among them may be found members adapted to feed- 

 ing, n ; others having purely sensory functions, called tentacu- 

 lar zooids, t; some adapted for reproduction, g; others in the form 

 of bells with muscles which enable them to move about, called 

 the swimming zooids, sw; and 

 finally at the top of the colony 



FIG. 72. CAMPANULARIA 



A colony of Hydroids showing a differenti- 

 ation into feeding zooids, nz, and generative 

 zooids, gz; e, e, eggs in different stages of 

 development; e' the young embryo extruded 

 into the water. 



FIG. 73. A SIPHONOPHORE 



An animal showing a high condi- 

 tion of polymorphism; /, the floating 

 zooid; g, the generative zooid ; n. the 

 nutritive zooid; sw, the swimming 

 zooid; t, the tentacular zooid. 



a single one develops as a gas bladder, /, which enables the 

 animal to float in the water. All of these combine to form 

 a colony. Where several different types are found arising by 

 budding from the same original stock the condition is spoken 

 of as polymorphism (Gr. polus = many + morphe = form). 

 Polymorphism is best illustrated in simple organisms, being well 

 developed among the animals related to the Hydra; but the sama 



