170 PRACTICAL ZOOLOGY. 



c. Besides these two are often found forms modified 

 for protection, etc. 



If a stained and mounted specimen of a campanu- 

 larian or other hydroid be at hand, it will be found 

 very useful in showing these points. 



The different kinds of individuals, though often 

 greatly modified, still show the essential plan of the 

 hydra. Some hydroids have a tube of hard material 

 developed by the outer layer, and at the base of the 

 colony some kinds secrete a layer of this material 

 incrusting the object on which the colony is borne. 

 Some forms spread by runners like strawberries. 

 One form is common on the shells inhabited by her- 

 mit crabs. Others are attached to seaweed, while still 

 others are dredged up from great depths of the ocean. 



Among certain forms of hydroids the generative 

 zooid becomes peculiarly modified in form, and ulti- 

 mately becoming detached, is known as a free gener- 

 ative zooid, jelly-fish, or medusa. These jelly-fishes, 

 or medusae are usually either bell-shaped or umbrella- 

 shaped, the part answering to the top being called the 

 bell or disk. Corresponding to a short handle is the 

 manubrium. This has at its free end an opening, 

 the mouth. The handle is hollow, and communi- 

 cates with tubes radiating through the disk, answer- 

 ing to the umbrella rays. These tubes are connected 

 by a circular tube, extending around the margin of 

 the disk. Along this margin are tentacles and organs 

 for receiving impressions of light or sound. Most 

 jelly-fishes swim by contracting the umbrella-like disk. 



Along the radiating tubes, or in the manubrium, 

 are borne the generative elements; the eggs develop 



