Coelenterata. 319 



The idea of community life is well carried out in that 

 there is a well-marked division of labor among the mem- 

 bers. Some, called nutritive individuals, devote themselves 

 to the work of obtaining and preparing food for the whole ; 

 some develop the stinging cells and protect the others ; 

 while still others are specialized for the work of reproduc- 

 tion, and depend on other members for nourishment and 

 protection. 



General Appearance of Hydroids. Most of the hydroids 

 are plantlike in appearance, hence are often called zoo- 

 phytes. Some resemble tufts of moss, others are simply 

 branched and trailing like the ground pine. They vary 

 greatly in color, from white to dark brown, while some are 

 of a beautiful pink. A form that will serve well for 

 example is whitish or brownish, and forms a downy or 

 furry coating on the wooden piles of wharves, piers, etc. 

 Many threads creep along the surface, while others rise at 

 right angles to the surface and end in budlike swellings. 

 Examined more closely, these terminal enlargements are 

 found to be bell-shaped. These are the individuals borne 

 on the connecting and supporting stalks. The unit, or 

 zooid, as it is called, is very much like a hydra. The body 

 is hollow, with a circle of arms surrounding the mouth, 

 which is at the free end. It is unlike the hydra in at least 

 three respects. First, the base, instead of being closed, 

 opens into the supporting tube, and this is in communica- 

 tion with all the other zooids of the colony. Second, the 

 tentacles are solid instead of hollow ; but they are like the 

 tentacles of the hydra in being flexible and provided with 

 stinging cells that serve for securing food. Third, the 

 mouth is distinctly raised above the bases of the tentacles, 

 and is capable of closing into a cone-shaped mass, the 

 hypostome, or opening into a bell-shaped entrance. 



